Optical pickup apparatus and optical element

ABSTRACT

An optical pickup apparatus comprises first, second and third light sources to emit light fluxes of wavelength λ 1, λ2  and λ 3  for conducting recording and/or reproducing information for first, second and third optical information recording mediums having respective protective substrates of thickness t 1 , t 2  and t 3  and a diffractive optical element located on a common optical path for the first, second and third light sources. A converged-light spot is formed on the first optical information recording medium with m-th order diffracted-light ray of the wavelength λ 1 , on the second optical information recording medium with n-th order diffracted-light ray of the wavelength λ 2 , and on the third optical information recording medium with k-th order diffracted-light ray of the wavelength λ 3  generated by the diffractive optical element respectively, wherein one of m, n and k is different from one of other two numbers.

RELATED APPLICATIONS

This is a division of application Ser. No. 10/651,679 filed Aug. 29,2003 now U.S. Pat. No. 7,577,077.

BACKGROUND OF THE INVENTION

The present invention relates to an optical pickup device and no anobjective optical element used for the optical pickup device, and inparticular, to an optical pickup device complying with standards of aplurality of optical information recording media (optical discs) and toan objective optical element used for the optical pickup device.

Up to the present, there have been developed and manufactured opticalpickup devices (which are also called optical heads or optical headdevices) for conducting reproducing and recording of information foroptical information recording media (which are also called optical discsor media) such as CD (compact disc) and DVD (digital video disc ordigital versatile disc), and they have come into wide use generally.

In recent years, there have been made studies and developments forstandards of optical information recording media which make high densityinformation recording possible.

The optical pickup device of this kind converges a light flux emittedfrom a light source (a laser diode is mainly used) on an informationrecording surface of an optical disc through an optical system (composedof optical elements such as a beam regulating prism, a collimator, abeam splitter and an objective optical element) to form a spot, andthen, converges reflected light from an information-recorded hole (whichis also called a pit) on a sensor through the optical system again toreproduce information by converting into electric signals. In this case,a light flux of the reflected light varies depending on a form of theinformation-recorded hole, and therefore, by using this, information of“0” is distinguished from that of “1”. Incidentally, on an informationrecording surface of the optical disc, there is provided a protectivesubstrate (a protective layer made of plastic which is also called acover glass).

Further, when recording information on a recording type medium such asCD-R or CD-RW, a spot by means of a laser light source is formed on arecording surface to generate thermochemical changes in a recordingmaterial on the recording surface. Due to this, in the case of CD-R,irreversible changes are caused on thermo-diffusive dyes, and a shapeidentical to the information-recorded hole is formed. In the case ofCD-RW wherein a phase change type material is used, reversible changesare made by thermochemical changes between the state of crystal and theamorphous state, and thereby, it is possible to rewrite information.

In the optical pickup device for reproducing information from an opticaldisc that is in compliance with CD standards, NA of an objective lens isabout 0.45 and a wavelength of a working light source is about 785 nm.For recording, on the other hand, those with about 0.50 are usuallyused. Incidentally, a thickness of a protective substrate of an opticaldisc complying with CD standards is 1.2 mm.

As an optical information recording medium, CD has been popularizedwidely, and in recent years, there has been diffused DVD. The DVD is onewherein a capacity for information storage has been increased by makinga protective substrate to be thinner and by making aninformation-recorded hole to be smaller, than those of CD, and arecording capacity of DVD is as large as about 4.7 GB (gigabyte) forthat of CD which is about 600-700 MB (megabyte), and DVD is used in manycases as a distributed medium wherein an animation image such as a movieis recorded.

In the optical pickup device for reproducing information from an opticaldisc that is in compliance with DVD standards, NA of an objective lensis about 0.60 and a wavelength of a working light source is about 655nm, because the information-recorded hole is small as stated above,although the principle of DVD is the same as that of CD. For recording,on the other hand, those with about 0.65 are usually used. Incidentally,a thickness of a protective substrate of an optical disc complying withDVD standards is 0.6 mm.

With respect to the optical disc in compliance with DVD standards, thoseof a recording type have already been put to practical use, and thereare respective standards including DVD-RAM, DVD-RW/R and DVD+RW/R.Technical principles for these are the same as those in the case of CDstandards.

An optical disc of a higher density and higher capacity type is nowbeing proposed as stated above.

This is one employing mainly the so-called violet laser light sourcehaving a wavelength of about 405 nm.

With respect to the “high density optical disc” of this kind, athickness of a protective substrate, a storage capacity and NA are notdetermined uniformly, even when a wording wavelength is determined.

If the direction for improving recording density substantially, athickness of a protective substrate of the optical disc is required tobe thin and NA is required to be great accordingly. On the contrary, itis also possible to make the thickness of a protective substrate and NAto be the same the standards of the conventional optical disc such asDVD. In that case, properties required as an optical system arerelatively generous, though physical recording density is not improvedsubstantially.

Specifically, with respect to a thickness of the protective substrate, athickness that is further thinned to be as thin as 0.1 mm and athickness of 0.6 mm that is identical to that of DVD have been proposed.

Though the plural standards for the “high density optical disc” of thiskind are the same in principle as those for CD and DVD, if a thicknessof the protective substrate is different, a size of theinformation-recorded hole is also different. Therefore, even if a lightsource having the same wavelength is used, it is not possible to conductreproducing and recording of information by using the same opticalpickup device simply.

Therefore, some problems need to be solved for attaining “compatibility”which makes it possible to conduct reproducing and recording ofinformation for both “high density optical disc” and conventional pluralstandards, with a single optical pickup device including an objectiveoptical element.

-   (1) Since an appropriate NA varies depending on each optical disc,    “diaphragm” functions to use respective NA selectively for media are    required.-   (2) On optical discs each having a different thickness of a    protective substrate, spherical aberrations are caused by the    thickness difference, which needs to be solved.

In particular, extremely remarkable spherical aberration is caused forthe difference between a thickness of 0.6 mm for DVD and that of 1.2 mmfor CD. Further, when a thickness of the protective substrate of “highdensity optical disc” is made to be 0.1 mm, further correction ofspherical aberration is needed.

For these problems, there have been proposed a method to use a dichroicfilter having a property to select wavelengths and a method to give aproperty to select wavelengths by providing a diffractive structure oran optical path difference furnishing structure on an objective opticalelement, and they have been realized.

There also exist the following problems.

-   (3) When attaining “compatibility” between standards of optical    discs wherein a thickness of a protective substrate and NA are the    same (including mostly the same) for both optical discs and a    working wavelength is different (for example, when using 655 nm and    405 nm), occurrence of spherical aberration (=spherical aberration    equivalent to an amount of chromatic aberration) caused by the    wavelength difference is unavoidable, although spherical aberration    caused by a difference of protective substrate thickness does not    occur, which needs to be solved.-   (4) With respect to light fluxes to enter an objective optical    element, when all light fluxes are collimated to be infinite    parallel light even when a wavelength is different, spherical    aberration based on a magnification difference is not caused, but in    the case of finite light (divergent light and convergent light), a    difference of magnification is caused, and spherical aberration    based on this difference of magnification is caused.-   (5) When attaining “compatibility” for three formats of CD, DVD and    “high density optical disc” by using a single optical pickup device,    it is necessary to conduct spherical aberration correction which is    more complicated than the occasion for “compatibility” between two    formats. In other words, corrections need to be made ‘between CD and    DVD’, ‘between CD and “high density optical disk”’ and ‘between DVD    and “high density optical disk”’, for the correction only between CD    and DVD in the case of two formats.

In Patent Document 1, there is disclosed a compatible optical pickupdevice that is compatible between the “high density optical disc” andDVD both mentioned in the present invention, which has an opticalelement having a diffractive structure in its optical path, and forms aconverged-light spots by diffracted light with different orders such assecond order diffraction for “high density optical disk” and first orderdiffraction for DVD or third order diffraction for “high density opticaldisk” and second order diffraction for DVD.

However, there is neither disclosure nor suggestion for technologiescoping with three types of formats such as those in the presentinvention.

(Patent Document 1)

TOKKAI No. 2001-93179

As stated above, for “compatibility” between different optical discs,there have been proposed a method to use a dichroic filter having aproperty to select wavelengths and a method to give a property to selectwavelengths by providing a diffractive structure on an objective opticalelement, and they have been realized.

However, there is a fear that optical performances are not attained whenan objective optical element is made to have various optical functions,although it is preferable for reduction of the number of parts, low costand downsizing.

When attaining the compatibility for three formats which is to be solvedthis time, it is impossible to solve by using simply the aforesaidmethods, because of many objects to be corrected.

Further, spherical aberration is sometimes caused also by themagnification of a light flux entering an objective optical element,which needs to be solved.

Since the optical pickup device itself is required to be small in size,light in weight and thin in thickness, factor parts, in particular,optical elements are required to have strict capabilities.

When the optical pickup device is made thin, in particular, a workingdistance (distance from an objective optical element to an optical disc)becomes short. Even if the working distance is made to be great byenhancing the magnification, image height characteristics are worsened,which is not preferable. If a difference of working distances is great,a load on an actuator is increased, and power consumption is alsoincreased.

In Patent Document 2, there is disclosed an optical disc device that isapproximately composed of the first-third light sources which emit lightrespectively of the first-third wavelengths, an objective lens thatreceives light of the first-third wavelengths respectively to convergethem on a prescribed optical information recording medium and of acollimator lens.

Light having respectively the first and the second wavelengths emittedrespectively from the first and the second light sources pass throughthe collimator lens, and in this case, light with the first wavelengthis collimated by the collimator lens to be parallel light and enters theobjective lens, while, light with the second wavelength is notcollimated and enters the objective lens as divergent light. Further,light with the third wavelength emitted from the third light source doesnot pass through the collimator lens and enters the objective lensdirectly as a divergent light.

Light each having one of the first-third wavelengths emerging from theobjective lens is converged on each of three types of opticalinformation recording media such as a high density optical disk, DVD andCD, each having a different wavelength and a different protectivesubstrate thickness, thus, recording and/or reproducing for varioustypes of information is conducted.

(Patent Document 2)

TOKKAI No. 2001-43559

Incidentally, in the apparatus disclosed in Patent Document 2, lightwith the first wavelength enters an objective lens as a parallel lightas stated above, while, light with the second wavelength and light withthe third wavelength enter the objective lens as divergent light, andthus, an optical system magnification of a light-converging opticalsystem is different from others for three types of optical informationrecording media.

Therefore, for example, an optical path for light with each of thefirst-third wavelengths is different from others, and therefore, aplurality of optical elements are required to be arranged so that eachof them may correspond to each optical path, and thereby, there havebeen problems that a structure of an optical disc device is complicatedand the number of parts of the apparatus is increased.

Further, there have been problems that image height characteristics areworsened and various aberrations such as coma and astigmatism arecaused, in the case of tracking to move an objective lens for an opticaldisc when conducting reproducing and recording for the optical disc,because divergent light enters the objective lens.

There has further been a problem that spherical aberration caused bytemperature changes is greater, compared with the so-called apparatus ofan infinity system type wherein parallel light enters an objective lens.

SUMMARY OF THE INVENTION

An object of the invention is to provide an optical pickup device and anoptical element to be used for reproducing and/or recording ofinformation for three types of optical information recording media eachhaving a different wavelength and a different protective substratethickness wherein the aforementioned problems are taken intoconsideration, various types of aberrations are inhibited, and thenumber of parts can be reduced.

(Item 1-1)

An optical pickup device conducting reproducing and/or recording ofinformation by using a light flux emitted from a first light sourcehaving wavelength λ1 for the first optical information recording mediumhaving protective substrate thickness t1, conducting reproducing and/orrecording of information by using a light flux emitted from a secondlight source having wavelength λ2 (λ1<λ2) for the second opticalinformation recording medium having protective substrate thickness t2(t1≦t2), and conducting reproducing and/or recording of information byusing a light flux emitted from a third light source having wavelengthλ3 (λ2<λ3) for the third optical information recording medium havingprotective substrate thickness t3 (t2<t3), wherein when conductingreproducing and/or recording of information for the first, second andthird optical information recording media, a light flux of infiniteparallel rays is made to enter an objective optical element included inthe optical pickup device, and the optical pickup device is providedwith a diffractive optical element that is arranged in a common opticalpath for the first, second and third light sources and has a diffractivestructure, and is constituted so that a converged-light spot is formedon the first optical information recording medium by the m^(th) orderdiffracted light (m is a natural number) generated by the diffractiveoptical element, and a converged-light spot is formed on the secondoptical information recording medium by the n^(th) order diffractedlight (n is a natural number satisfying n≠m) generated by thediffractive optical element.

(Item 1-2)

The optical pickup device described in Item 1-1 wherein the diffractiveoptical element is the objective optical element.

(Item 1-3)

The optical pickup device described in Item 1-1 wherein the diffractiveoptical element is a collimator.

(Item 1-4)

The optical pickup device described in Item 1-1 wherein the diffractiveoptical element is an optical element provided separately from theobjective optical element and the collimator.

(Item 1-5)

An optical pickup device conducting reproducing and/or recording ofinformation by using a light flux emitted from a first light sourcehaving wavelength λ1 for the first optical information recording mediumhaving protective substrate thickness t1, conducting reproducing and/orrecording of information by using a light flux emitted from a secondlight source having wavelength λ2 (λ1<λ2) for the second opticalinformation recording medium having protective substrate thickness t2(t1≦t2), and conducting reproducing and/or recording of information byusing a light flux emitted from a third light source having wavelengthλ3 (λ2<λ3) for the third optical information recording medium havingprotective substrate thickness t3 (t2≦t3), wherein the optical pickupdevice is provided with a first compatible optical element arranged in acommon optical path for the first, second and third light sources andwith a second compatible optical element arranged in an optical path forone light source among the first, second and third light sources or in acommon optical path for certain two light sources, and the firstcompatible optical element has a first compatible function for forming aconverged-light spot necessary for conducting reproducing and/orrecording of information for at least one optical information recordingmedium among the first, second and third optical information recordingmedia, and the second compatible optical element has, by combining withthe first compatible optical element, a second compatible function forforming a converged-light spot necessary for conducting reproducingand/or recording of information for other optical information recordingmedia among the first, second and third optical information recordingmedia, and the optical pickup device is provided with a diffractiveoptical element that is arranged in a common optical path for the first,second and third light sources and has a diffractive structure, and isconstituted so that a converged-light spot is formed on the firstoptical information recording medium by the m^(th) order diffractedlight (m is a natural number) generated by the diffractive opticalelement, and a converged-light spot is formed on the second opticalinformation recording medium by the n^(th) order diffracted light (n isa natural number satisfying n≠m) generated by the diffractive opticalelement.

(Item 1-6)

The optical pickup device described in Item 1-5, wherein the firstcompatible optical element is an objective optical element.

(Item 1-7)

The optical pickup device described in Item 1-5, wherein the secondcompatible optical element is a dichroic filter.

(Item 1-8)

The optical pickup device described in either one of Item 1-5 and Item1-6 wherein the second compatible optical element is a liquid crystalelement.

(Item 1-9)

The optical pickup device described in either one of Item 1-5 and Item1-6 wherein the second compatible optical element is a diffractiveoptical element.

(Item 1-10)

The optical pickup device described in either one of Item 1-5-Item 1-9wherein, when conducting reproducing and/or recording of information foroptical information recording media, a light flux having the samemagnification is made to enter an objective optical element for all ofthe first, second and third optical information recording media, and thefirst and second compatible functions correct spherical aberration basedon a wavelength difference and spherical aberration based on adifference of protective substrate thickness between optical informationrecording media.

(Item 1-11)

The optical pickup device described in either one of Item 1-5-Item 1-9wherein, when conducting reproducing and/or recording of information foroptical information recording media, a light flux having the differentmagnification is made to enter an objective optical element for all ofthe first, second and third optical information recording media, and thefirst and second compatible functions correct spherical aberration basedon a wavelength difference, spherical aberration based on a differenceof protective substrate thickness between optical information recordingmedia and spherical aberration based on a difference of magnification ofthe light flux entering the objective optical element.

(Item 1-12)

The optical pickup device described in either one of Item 1-1-Item 1-11wherein m is equal to 2.

(Item 1-13)

The optical pickup device described in either one of Item 1-1-Item 1-12wherein n is equal to 1.

(Item 1-14)

The optical pickup device described in either one of Item 1-1-Item 1-13wherein a converged-light spot by n^(th) order diffracted lightgenerated by the diffractive optical element is formed on the thirdoptical information recording medium.

(Item 1-15)

The optical pickup device described in either one of Item 1-1-Item 1-14wherein there is provided an optical correcting structure for conductingtemperature compensation and/or chromatic aberration compensation.

(Item 1-16)

An optical pickup device conducting reproducing and/or recording ofinformation by using a light flux emitted from a first light sourcehaving wavelength λ1 for the first optical information recording mediumhaving protective substrate thickness t1, conducting reproducing and/orrecording of information by using a light flux emitted from a secondlight source having wavelength λ2 (λ1<λ2) for the second opticalinformation recording medium having protective substrate thickness t2(t1≦t2), and conducting reproducing and/or recording of information byusing a light flux emitted from a third light source having wavelengthλ3 (λ2<λ3) for the third optical information recording medium havingprotective substrate thickness t3 (t2≦t3), wherein the optical pickupdevice is equipped with a diffractive optical element arranged in acommon optical path for the first, second and third light sources andwith a compatible optical element which is arranged to be closer to alight source than the diffractive optical element is, and can switchoptical function for each wavelength, and when conducting reproducingand/or recording of information for the first, second and third opticalinformation recording media, a light flux of infinite parallel rays ismade to enter the compatible optical element, while, the diffractiveoptical element forms a converged-light spot that is sufficient forconducting reproducing and/or recording of information on at least thefirst optical information recording medium and generates diffractedlight with an order different from that of the light flux with λ1 forthe light flux with λ2 or the light flux with λ3, and the compatibleoptical element generates an optical function that is different from theoptical function of the light flux with λ1 on the second opticalinformation recording medium and the third optical information recordingmedium and forms, when combined with the optical function of thediffractive optical element, a converged-light spot that is sufficientfor conducting reproducing and/or recording of information on the secondoptical information recording medium and the third optical informationrecording medium.

(Item 1-17)

An optical pickup device conducting reproducing and/or recording ofinformation by using a light flux emitted from a first light sourcehaving wavelength λ1 for the first optical information recording mediumhaving protective substrate thickness t1, conducting reproducing and/orrecording of information by using a light flux emitted from a secondlight source having wavelength λ2 (λ1<λ2) for the second opticalinformation recording medium having protective substrate thickness t2(t1≦t2), and conducting reproducing and/or recording of information byusing a light flux emitted from a third light source having wavelengthλ3 (λ2<λ3) for the third optical information recording medium havingprotective substrate thickness t3 (t2≦t3), wherein the optical pickupdevice is equipped with a diffractive optical element arranged in acommon optical path for the first, second and third light sources andwith a compatible optical element which is arranged to be closer to alight source than the diffractive optical element is, and can switchoptical function for each wavelength, and when conducting reproducingand/or recording of information for the first, second and third opticalinformation recording media, a light flux of infinite parallel rays ismade to enter the compatible optical element, while, the diffractiveoptical element forms a converged-light spot necessary for conductingreproducing and/or recording of information by means of diffracted lighteach having a different diffraction order, on two optical informationrecording media among the first, second and third optical informationrecording media, and the compatible optical element generates an opticalfunction that is different from the optical function of the light fluxwith λ1 on the second optical information recording medium and the thirdoptical information recording medium, and forms, when combined with theoptical function of the diffractive optical element, a converged-lightspot that is necessary for conducting reproducing and/or recording ofinformation on the other optical information recording medium among thesecond optical information recording medium and the third opticalinformation recording medium.

(Item 1-18)

The optical pickup device described in either one of Item 1-16 and Item1-17 wherein the diffractive optical element is an objective opticalelement.

(Item 1-19)

The optical pickup device described in Item 1-18 wherein the objectiveoptical element is a single lens.

(Item 1-20)

The optical pickup device described in Item 1-18 wherein the objectiveoptical element is a multi-lens.

(Item 1-21)

The optical pickup device described in either one of Items 1-16-1-20wherein the compatible optical element does not generate opticalfunction on the light flux with λ1.

(Item 1-22)

The optical pickup device described in either one of Item 1-s 16-20wherein the compatible optical element is a liquid crystal element.

(Item 1-23)

The optical pickup device described in Item 1-22 wherein opticalfunctions are switched by making the state of energization to bedifferent by a wavelength of a light flux entering the liquid crystalelement.

(Item 1-24)

The optical pickup device described in either one of Item 1-s 16-20wherein the compatible optical element is a movable beam expander.

(Item 1-25)

The optical pickup device described in Item 1-24 wherein opticalfunctions are switched by moving the beam expander in the direction ofan optical axis depending on a wavelength of an entering light flux.

(Item 1-26)

The optical pickup device described in either one of Items 1-16-1-25wherein the diffractive optical element and the compatible opticalelement are unified to be held and driven by a single driving means.

(Item 1-27)

The optical pickup device described in either one of Item 1-s 16-26wherein the diffracting surface is of the multi-level structure.

(Item 1-28)

The optical pickup device described in either one of Item 1-s 16 andItem 1-s 18-27 wherein the diffractive optical element forms aconverged-light spot which is insufficient for conducting reproducingand/or recording of information on the second and third opticalinformation recording media.

(Item 1-29)

The optical pickup device described in Item 1-16 wherein k^(th) order (kis a natural number) diffracted light is generated for the light fluxwith λ1, m^(th) order (m is a natural number satisfying m≠k) diffractedlight is generated for the light flux with λ2 and n^(th) order (n is anatural number satisfying n≠k) diffracted light is generated for thelight flux with λ1, all by the diffractive optical element.

(Item 1-30)

The optical pickup device described in Item 1-18 wherein m≠n holds.

(Item 1-31)

The optical pickup device described in Item 1-18 wherein m=n holds.

(Item 1-32)

The optical pickup device described in Item 1-30 wherein k=1, m=0 andn=2 hold.

(Item 1-33)

The optical pickup device described in Item 1-31 wherein k=2, m=1 andn=1 hold.

(Item 1-34)

The optical pickup device described in Item 1-30 wherein k=2, m=1 andn=0 hold.

(Item 1-35)

The optical pickup device described in Item 1-30 wherein k=2, m=2 andn=1 hold.

(Item 1-36)

The optical pickup device described in Item 1-31 wherein k=3, m=2 andn=2 hold.

(Item 1-37)

The optical pickup device described in Item 1-30 wherein k=4, m=3 andn=2 hold.

(Item 1-38)

The optical pickup device described in Item 1-30 wherein k=5, m=3 andn=2 hold.

(Item 1-39)

The optical pickup device described in Item 1-31 wherein k=5, m=3 andn=3 hold.

(Item 1-40)

The optical pickup device described in Item 1-30 wherein k=6, m=4 andn=3 hold.

(Item 1-41)

The optical pickup device described in Item 1-31 wherein k=7, m=4 andn=4 hold.

(Item 1-42)

The optical pickup device described in Item 1-30 wherein k=8, m=5 andn=4 hold.

(Item 1-43)

The optical pickup device described in Item 1-17 wherein the diffractiveoptical element has a diffracting surface that forms, using diffractedlight each having a different order number, converged-light spotsnecessary for conducting reproducing and/or recording of information onthe first and second optical information recording media.

(Item 1-44)

The optical pickup device described in Item 1-43 wherein the diffractingsurface is provided on the total area of the optical functional surfaceof the diffractive optical element, and the diffracting surface correctsspherical aberration based on a wavelength difference between thewavelength λ1 and the wavelength λ2.

(Item 1-45)

The optical pickup device described in either one of Item 1-43 and Item1-44 wherein the compatible optical element corrects sphericalaberration caused by a difference of substrate thickness betweensubstrate thickness t1 and substrate thickness t3 and sphericalaberration caused by a wavelength difference between the wavelength λ1and the wavelength λ3.

(Item 1-46)

The optical pickup device described in either one of Items 1-43-1-45wherein k^(th) (k is a natural number) order diffracted light isgenerated for the light flux with λ1, m^(th) (m is a natural numbersatisfying m≠) order diffracted light is generated for the light fluxwith λ2, and n^(th) (n is a natural number satisfying n≠k) orderdiffracted light is generated for the light flux with λ1, all by thediffractive optical element.

(Item 1-47)

The optical pickup device described in Item 1-46 wherein m≠n holds.

(Item 1-48)

The optical pickup device described in Item 1-46 wherein m=n holds.

(Item 1-49)

The optical pickup device described in Item 1-47 wherein k=1, m=0 andn=2 hold.

(Item 1-50)

The optical pickup device described in Item 1-48 wherein k=2, m=1 andn=1 hold.

(Item 1-51)

The optical pickup device described in Item 1-47 wherein k=2, m=1 andn=0 hold.

(Item 1-52)

The optical pickup device described in Item 1-47 wherein k=2, m=2 andn=1 hold.

(Item 1-53)

The optical pickup device described in Item 1-48 wherein k=3, m=2 andn=2 hold.

(Item 1-54)

The optical pickup device described in Item 1-47 wherein k=4, m=3 andn=2 hold.

(Item 1-55)

The optical pickup device described in Item 1-47 wherein k=5, m=3 andn=2 hold.

(Item 1-56)

The optical pickup device described in Item 1-48 wherein k=5, m=3 andn=3 hold.

(Item 1-57)

The optical pickup device described in Item 1-47 wherein k=6, m=4 andn=3 hold.

(Item 1-58)

The optical pickup device described in Item 1-48 wherein k=7, m=4 andn=4 hold.

(Item 1-59)

The optical pickup device described in Item 1-47 wherein k=8, m=5 andn=4 hold.

(Item 1-60)

The optical pickup device described in Item 1-17 wherein the diffractiveoptical element has a diffracting surface that forms, using diffractedlight each having a different order number, converged-light spotsnecessary for conducting reproducing and/or recording of information onthe first and third optical information recording media.

(Item 1-61)

The optical pickup device described in Item 1-60 wherein the diffractingsurface is provided on the total area of the optical functional surfaceof the diffractive optical element, and the diffracting surface correctsspherical aberration based on a substrate thickness difference betweensubstrate thickness t1 and substrate thickness t3 and sphericalaberration based on a wavelength difference between the wavelength λ1and the wavelength λ3.

(Item 1-62)

The optical pickup device described in either one of Item 1-s 60 and 61wherein the compatible optical element corrects spherical aberrationbased on a wavelength difference between the wavelength λ1 and thewavelength λ2.

(Item 1-63)

The optical pickup device described in either one of Item 1-s 60-62wherein k^(th) (k is a natural number) order diffracted light isgenerated for the light flux with λ1, m^(th) (m is a natural numbersatisfying m≠k) order diffracted light is generated for the light fluxwith λ2, and n^(th) (n is a natural number satisfying n≠k) orderdiffracted light is generated for the light flux with λ1, all by thediffractive optical element.

(Item 1-64)

The optical pickup device described in Item 1-63 wherein m≠n holds.

(Item 1-65)

The optical pickup device described in Item 1-63 wherein m=n holds.

(Item 1-66)

The optical pickup device described in Item 1-64 wherein k=1, m=0 andn=2 hold.

(Item 1-67)

The optical pickup device described in Item 1-64 wherein k=2, m=1 andn=0 hold.

(Item 1-68)

The optical pickup device described in Item 1-65 wherein k=3, m=2 andn=2 hold.

(Item 1-69)

The optical pickup device described in Item 1-64 wherein k=5, m=3 andn=2 hold.

(Item 1-70)

The optical pickup device described in Item 1-65 wherein k=5, m=3 andn=3 hold.

(Item 1-71)

The optical pickup device described in Item 1-65 wherein k=7, m=4 andn=4 hold.

Item 2-1

To solve the aforementioned problems, the invention described in Item2-1 is an optical pickup device to conduct reproducing and/or recordingof information by using a light flux emitted from the first light sourcewith wavelength λ1 on the first optical information recording mediumhaving protective substrate thickness t1, to conduct reproducing and/orrecording of information by using a light flux emitted from the secondlight source with wavelength λ2 (λ1<λ2) on the second opticalinformation recording medium having protective substrate thickness t2(t1≦t2), and to conduct reproducing and/or recording of information byusing a light flux emitted from the third light source with wavelengthλ3 (λ2<λ3) on the third optical information recording medium havingprotective substrate thickness t3 (t2≦t3), wherein the optical pickupdevice is arranged on the common optical path for the first, second andthird light sources, and is equipped with a diffractive optical elementhaving the first diffractive structure, and is structured so that alllight fluxes are made to enter the diffracting light optical elementincluded in the optical pickup device at the substantially same angle, aconverged-light spot of the m^(th) (m represents a natural number) orderdiffracted light is formed by the diffractive optical element on thefirst optical information recording medium and a converged-light spot ofthe n^(th) (n represents a natural number satisfying n≠m) orderdiffracted light is formed by the diffractive optical element on thesecond optical information recording medium, when conducting reproducingand/or recording of information for the first, second and third opticalinformation recording media.

In the invention described in Item 2-1, the optical pickup device isarranged on the common optical path for the first, second and thirdlight sources, and a diffractive optical element having the firstdiffractive structure is provided, and when conducting reproducingand/or recording of information for the first, second and third opticalinformation recording media, all light fluxes are made to enter thediffracting light optical element at the substantially same angle.

Therefore, the optical paths for light respectively with the first-thirdwavelengths are mostly the same, thus, various types of optical elementsconstituting the optical pickup device have only to be arranged tocorrespond to the common optical path, thereby, the structure of theoptical pickup device can be simplified and the number of parts of thedevice can be reduced.

Item 2-2

The invention described in Item 2-2 is the optical pickup deviceaccording to Item 2-1, wherein the diffractive optical element is anobjective optical element.

Item 2-3

The invention described in Item 2-3 is the optical pickup deviceaccording to Item 2-2, wherein all the light fluxes stated above aremade to enter the diffractive optical element as substantially infiniteparallel rays.

In the invention described in Item 2-3, the same effect as in Item 2-2can be obtained and all the light fluxes are made to enter thediffractive optical element as substantially infinite parallel rays.

Therefore, it is possible to prevent that image height characteristicsare worsened in the case of tracking to move an objective opticalelement for the optical information recording medium, and to inhibitoccurrence of various aberrations such as coma and astigmatism.

It is further possible to inhibit spherical aberration caused bytemperature changes.

Item 2-4

The invention described in Item 2-4 is the optical pickup deviceaccording to either one of Item 2-s 1-3 wherein the diffractive opticalelement functions as a collimator when the light flux having wavelengthλ1 enters.

Item 2-5

The invention described in Item 2-5 is the optical pickup deviceaccording to either one of Items 2-1-3 wherein the diffractive opticalelement functions as a collimator when the light flux having wavelengthλ2 enters.

Item 2-6

The invention described in Item 2-6 is the optical pickup deviceaccording to Item 2-1, wherein the diffractive optical element mentionedabove is an optical element provided separately from the objectiveoptical element and the collimator which constitute the optical pickupdevice.

Item 2-7

The invention described in Item 2-7 is an optical pickup device toconduct reproducing and/or recording of information by using a lightflux emitted from the first light source with wavelength λ1 on the firstoptical information recording medium having protective substratethickness t1, to conduct reproducing and/or recording of information byusing a light flux emitted from the second light source with wavelengthλ2 (λ1<λ2) on the second optical information recording medium havingprotective substrate thickness t2 (t1<t2), and to conduct reproducingand/or recording of information by using a light flux emitted from thethird light source with wavelength λ3 (λ2<λ3) on the third opticalinformation recording medium having protective substrate thickness t3(t2≦t3), wherein the optical pickup device is equipped with a firstcompatible optical element that is arranged in a common optical path forthe first, second and third light sources and with a second compatibleoptical element that is arranged in an optical path for only one of thefirst, second and third light sources, or in a common optical path forsome two light sources, and the first compatible optical element has afirst compatible function to form a converged-light spot necessary forconducting reproducing and/or recording of information for at least oneof the first, second and third optical information recording media,while the second compatible optical element has a second compatiblefunction, when it is combined with the first compatible optical element,to form a converged-light spot necessary for conducting reproducingand/or recording of information for the other optical informationrecording medium among the first, second and third optical informationrecording media, and the optical pickup device is arranged on the commonoptical path for the first, second and third light sources, and isequipped with a diffractive optical element having the first diffractivestructure, and a plurality of ring-shaped zonal optical surfaces with anoptical axis as the center are formed on at least one optical surface ofat least one optical element among the first compatible optical element,the second compatible optical element and the diffractive opticalelement, and the ring-shaped zonal optical surfaces are formedcontinuously through stepped surfaces, thus, a converged-light spot ofthe m^(th) (m represents a natural number) order diffracted light isformed by the diffractive optical element on the first opticalinformation recording medium and a converged-light spot of the n^(th) (nrepresents a natural number satisfying n≠m) order diffracted light isformed by the diffractive optical element on the second opticalinformation recording medium.

Item 2-8

The invention described in Item 2-8 is the optical pickup deviceaccording to Item 2-7, wherein the first compatible optical element isan objective optical element.

Item 2-9

The invention described in Item 2-9 is the optical pickup deviceaccording to Item 2-7 or Item 2-8 wherein the second compatible opticalelement is a phase difference plate.

Item 2-10

The invention described in Item 2-10 is the optical pickup deviceaccording to Item 2-7 or Item 2-8 wherein the second compatible opticalelement is a liquid crystal element.

Item 2-11

The invention described in Item 2-11 is the optical pickup deviceaccording to Item 2-7 or Item 2-8 wherein the second compatible opticalelement is a diffractive optical element.

Item 2-12

The invention described in Item 2-12 is the optical pickup deviceaccording to either one of Item 2-7-2-11, wherein the light-convergingoptical system that is composed of the first compatible optical element,the second compatible optical element, the diffractive optical elementand the objective optical element constituting the optical pickupdevice, has an optical system magnification that is substantially thesame for light fluxes having respectively the wavelengths λ1, λ2 and λ3,and the first compatible function and the second compatible functioncorrect spherical aberrations caused by a wavelength difference and by adifference of a protective substrate thickness between opticalinformation recording media.

Item 2-13

The invention described in Item 2-13 is the optical pickup deviceaccording to Item 2-12, wherein the optical system magnification issubstantially zero.

Item 2-14

The invention described in Item 2-14 is the optical pickup deviceaccording to either one of Items 2-7-2-11, wherein the light-convergingoptical system that is composed of the first compatible optical element,the second compatible optical element, the diffractive optical elementand the objective optical element constituting the optical pickupdevice, has optical system magnifications which are different for lightfluxes having respectively the wavelengths λ1, λ2 and λ3, and the firstcompatible function and the second compatible function correct sphericalaberrations caused by a wavelength difference, a difference of aprotective substrate thickness between optical information recordingmedia and by a difference in optical system magnifications of thelight-converging optical system.

Item 2-15

The invention described in Item 2-15 is the optical pickup deviceaccording to either one of Items 2-1-2-14, wherein there is provided anoptical correcting element for conducting temperature compensationand/or chromatic aberration compensation for at least oneconverged-light spot among converged-light spots formed on the first,second and third optical information recording media.

Item 2-16

The invention described in Item 2-16 is the optical pickup deviceaccording to Item 2-1 or Item 2-7, wherein NA3<NA1 and NA3<NA2 aresatisfied when NA1 represents a numerical aperture of a converged-lightspot formed on the first optical information recording medium by thelight flux having wavelength λ1, NA2 represents a numerical aperture ofa converged-light spot formed on the second optical informationrecording medium by the light flux having wavelength λ2, and NA3represents a numerical aperture of a converged-light spot formed on thethird optical information recording medium by the light flux havingwavelength λ3.

Item 2-17

The invention described in Item 2-17 is the optical pickup deviceaccording to Item 2-16, wherein a plurality of ring-shaped zonal opticalsurfaces are represented by at least one optical surface of at least oneoptical element among the first compatible optical element, the secondcompatible optical element and the diffractive optical element, and whena light flux with wavelength λ3 that forms a converged-light spot havingnumerical aperture NA3 on the third optical information recording mediumis formed at an area through which the light flux passes, and when Rsrepresents the ring-shaped zonal optical surface including an opticalaxis among the plural ring-shaped zonal optical surfaces, and R1represents the ring-shaped zonal optical surface which is farthest fromthe optical axis, light fluxes having respectively wavelengths λ1, λ2and λ3 which have passed through the ring-shaped zonal optical surfacesRs are used for reproducing and/or recording for respective opticalinformation recording media, while, a light flux with wavelength λ3 thathas passed through the ring-shaped zonal optical surface R1 is used forreproducing and/or recording for the third optical information recordingmedium.

Item 2-18

The invention described in Item 2-18 is the optical pickup deviceaccording to Item 2-17, wherein the first diffractive structure isformed on the area which is at least one optical surface of thediffractive optical element and through which the light flux with thewavelength λ3 which forms a light-converted spot with numerical apertureNA3 on the third optical information recording medium, and aconverged-light spot is formed on the third optical informationrecording medium by k^(th) (k is a natural number) order diffractedlight the light flux with wavelength λ3 generated by the firstdiffractive structure, satisfying k=m/2, 370 nm≦λ1≦430 nm and 760nm≦λ3≦810 nm.

Item 2-19

The invention described in Item 2-19 is the optical pickup deviceaccording to Item 2-17 or Item 2-18, wherein each of light fluxes havingrespectively wavelengths λ1 and λ2 which have passed through thering-shaped zonal optical surface Rs is converged on an informationrecording surface of each optical information recording medium, to besubstantially free from any aberrations.

Item 2-20

The invention described in Item 2-20 is the optical pickup deviceaccording to Item 2-18 or Item 2-19, wherein the ring-shaped zonaloptical surface and the first diffractive structure are formed on thesame surface of the diffractive optical element.

Item 2-21

The invention described in Item 2-21 is the optical pickup deviceaccording to either one of Items 2-17-2-20, wherein both of thediffraction efficiency of the m^(th) order diffracted light and that ofthe n^(th) order diffracted light are 80% or more.

Item 2-22

The invention described in Item 2-22 is the optical pickup deviceaccording to either one of Items 2-17-2-21, wherein the diffractionefficiency of the k^(th) order diffracted light is 50% or more.

Item 2-23

The invention described in Item 2-23 is the optical pickup deviceaccording to either one of Items 2-17-2-22, wherein a wave-frontaberration of the converged-light spot formed on the third opticalinformation recording medium by the light flux with wavelength λ3 is0.040 (λ3 rms) or less.

Item 2-24

The invention described in Item 2-24 is the optical pickup deviceaccording to either one of Items 2-17-2-23, wherein paraxial rays of thelight flux with wavelength λ3 are converged on a point which is closerto the light source than the position in the direction of an opticalaxis which makes a wave-front aberration of the converged-light spotformed on the third optical information recording medium by the lightflux with wavelength λ3 to be minimum.

Item 2-25

The invention described in Item 2-25 is the optical pickup deviceaccording to either one of Items 2-17-24, wherein the light fluxeshaving respectively wavelengths λ1 and λ2 enter the diffractive opticalelement at the same angle of divergence, or enter as the same infinitelight, and the first diffractive structure makes the diffraction effectby a difference between the wavelengths λ1 and λ2 to correct sphericalaberration caused by a difference between the wavelengths λ1 and λ2 andby refraction function of the optical surface on which the firstdiffractive structure is provided and spherical aberration caused by adifference between protective substrate thickness t1 and t2.

Item 2-26

The invention described in Item 2-26 is the optical pickup deviceaccording to either one of Items 2-17-25, wherein m=8 and n=5 aresatisfied.

Item 2-27

The invention described in Item 2-27 is the optical pickup deviceaccording to either one of Items 2-17-25, wherein m=6 and n=4 aresatisfied.

Item 2-28

The invention described in Item 2-28 is the optical pickup deviceaccording to either one of Items 2-17-2-25, wherein m=2 and n=1 aresatisfied.

Item 2-29

The invention described in Item 2-29 is the optical pickup deviceaccording to either one of Items 2-17-28, wherein 1.9×λ1≦λ3≦2.1×λ1 issatisfied.

Item 2-30

The invention described in Item 2-30 is the optical pickup deviceaccording to either one of Items 2-18-2-29, wherein the light flux withwavelength λ3 which has passed the ring-shaped optical surface Rs andthe light flux with wavelength λ3 which has passed the ring-shapedoptical surface R1 are converged to be away each other by 10 nm or morein the direction of an optical axis.

Item 2-31

The invention described in Item 2-31 is the optical pickup deviceaccording to Item 2-30, wherein −0.1π≦φ≦0.1π is satisfied by phasedifference φ at the converged-light spots for the light flux withwavelength λ3 which has passed through the ring-shaped optical surfaceRs and the light flux with wavelength λ3 which has passed through thering-shaped optical surface other than the ring-shaped optical surfaceRs.

Item 2-32

The invention described in Item 2-32 is the optical pickup deviceaccording to Item 2-30 or Item 2-31, wherein a phase differenceconcerning the light flux with wavelength λ3 is varied before and afterthe light flux passes through the adjoining ring-shaped zonal opticalsurfaces.

Item 2-33

The invention described in Item 2-33 is the optical pickup deviceaccording to either one of Items 2-30-2-32, wherein the phase differenceof at least one of the light fluxes respectively with wavelengths λ1 andλ2 is not varied before and after the light flux passes through theadjoining ring-shaped zonal optical surfaces.

Item 2-34

The invention described in Item 2-34 is the optical pickup deviceaccording to either one of Items 2-30-2-33, wherein the number of thering-shaped zonal optical surfaces is any number within a range from 2to 10.

Item 2-35

The invention described in Item 2-35 is the optical pickup deviceaccording to either one of Items 2-18-2-29, wherein the firstdiffractive structure is formed on an area through which the lightfluxes having respectively wavelengths λ1, λ2 and λ3 which formconverged-light spots on respective optical information recording mediaafter passing through the ring-shaped zonal optical surface Rs pass, andlight-converging position fB3 of the light flux with wavelength λ3 thatforms a converged-light spot after passing through the ring-shaped zonaloptical surface R1 satisfies |fB3|≦5 μm in the direction of an opticalaxis for the best image surface for the converged-light spot that isformed by the light flux with wavelength λ3 on the third opticalinformation recording medium.

Item 2-36

The invention described in Item 2-36 is the optical pickup deviceaccording to Item 2-35, wherein an area on the optical surface on whichthe first diffractive structure is formed through which the light fluxeshaving respectively wavelengths λ1, λ2 and λ3 which form converged-lightspots respectively on optical information recording media after passingthrough the ring-shaped zonal optical surface R1 pass is a refractinginterface.

Item 2-37

The invention described in Item 2-37 is the optical pickup deviceaccording to Item 2-35, wherein the second diffractive structure isformed on an area on the optical surface on which the first diffractivestructure is formed through which the light fluxes having respectivelywavelengths λ1, λ2 and λ3 which form converged-light spots respectivelyon optical information recording media after passing through thering-shaped zonal optical surface R1 pass.

Item 2-38

The invention described in Item 2-38 is the optical pickup deviceaccording to Item 2-37, wherein a combination of diffracted light thatgenerates the maximum diffraction efficiency among diffracted light ofrespective light fluxes generated by the first diffractive structurewhen light fluxes having respectively wavelengths λ1, λ2 and λ3 enter isdifferent from a combination of diffracted light that generates themaximum diffraction efficiency among diffracted light of respectivelight fluxes generated by the second diffractive structure.

Item 2-39

The invention described in Item 2-39 is the optical pickup deviceaccording to Item 2-38, wherein a combination of diffracted light thatgenerates the maximum diffraction efficiency among diffracted light ofrespective light fluxes generated by the second diffractive structurewhen light fluxes having respectively wavelengths λ1, λ2 and λ3 enter isa combination of 1, 1 and 1.

Item 2-40

The invention described in Item 2-40 is the optical pickup deviceaccording to either one of Items 2-35-2-39, wherein the light flux withwavelength λ1 which has passed through ring-shaped zonal optical surfaceR1 is converged on an information recording surface of the first opticalinformation recording medium, to be substantially free from anyaberrations.

Item 2-41

The invention described in Item 2-41 is the optical pickup deviceaccording to either one of Items 2-35-2-40, wherein the light flux withwavelength λ2 which has passed through ring-shaped zonal optical surfaceR1 is converged on an information recording surface of the secondoptical information recording medium, to be substantially free from anyaberrations.

Item 2-42

The invention described in Item 2-42 is the optical pickup deviceaccording to either one of Items 2-35-2-41, wherein a length in thedirection that is in parallel with an optical axis of the steppedsurface closer to the optical axis among two stepped surfaces followingthe ring-shaped zonal optical surface R1 is shorter than a length in thedirection that is in parallel with an optical axis of the other steppedsurface.

Item 2-43

The invention described in Item 2-43 is the optical pickup deviceaccording to either one of Item 2-35-2-42, wherein the number of thering-shaped zonal optical surfaces is two.

Item 2-44

The invention described in Item 2-44 represents a plurality of opticalelements included in an optical pickup device to conduct reproducingand/or recording of information by using a light flux emitted from thefirst light source with wavelength λ1 on the first optical informationrecording medium having protective substrate thickness t1, to conductreproducing and/or recording of information by using a light fluxemitted from the second light source with wavelength λ2 (λ1<λ2) on thefirst optical information recording medium having protective substratethickness t2 (t1≦t2), and to conduct reproducing and/or recording ofinformation by using a light flux emitted from the second light sourcewith wavelength λ3 (λ2<λ3) on the first optical information recordingmedium having protective substrate thickness t3 (t2≦t3), wherein theoptical pickup device is arranged on the common optical path for thefirst, second and third light sources, and it includes a diffractiveoptical element having the first diffractive structure, and isstructured so that all light fluxes are made to enter the diffractinglight optical element included in the optical pickup device at thesubstantially same angle, a converged-light spot of the m^(th) (mrepresents a natural number) order diffracted light is formed by thediffractive optical element on the first optical information recordingmedium and a converged-light spot of the n^(th) (n represents a naturalnumber satisfying n≠m) order diffracted light is formed by thediffractive optical element on the second optical information recordingmedium, when conducting reproducing and/or recording of information forthe first, second and third optical information recording media.

Item 2-45

The invention described in Item 2-45 is the optical element according toItem 2-44, wherein the diffractive optical element is an objectiveoptical element.

Item 2-46

The invention described in Item 2-46 is the optical element according toItem 2-45, wherein all the light fluxes stated above are made to enterthe diffractive optical element as substantially infinite parallel rays.

Item 2-47

The invention described in Item 2-47 is the optical element according toeither one of Item 2-44-2-46, wherein the diffractive optical elementfunctions as a collimator when the light flux having wavelength λ1enters.

Item 2-48

The invention described in Item 2-48 is the optical element according toeither one of Items 2-44-2-46, wherein the diffractive optical elementfunctions as a collimator when the light flux having wavelength λ2enters.

Item 2-49

The invention described in Item 2-49 is the optical element according toItem 2-44, wherein the diffractive optical element is providedseparately from an objective optical element and a collimator whichconstitute an optical pickup device.

Item 2-50

The invention described in Item 2-50 represents a plurality of opticalelements included in an optical pickup device to conduct reproducingand/or recording of information by using a light flux emitted from thefirst light source with wavelength λ1 on the first optical informationrecording medium having protective substrate thickness t1, to conductreproducing and/or recording of information by using a light fluxemitted from the second light source with wavelength λ2 (λ1<λ2) on thefirst optical information recording medium having protective substratethickness t2 (t1≦t2), and to conduct reproducing and/or recording ofinformation by using a light flux emitted from the second light sourcewith wavelength λ3 (λ2<λ3) on the first optical information recordingmedium having protective substrate thickness t3 (t2≦t3), wherein thefirst compatible optical element arranged on a common optical path ofthe first, second and third light sources and the second compatibleoptical element arranged on an optical path of only one of the first,second and third light sources or on a common optical path of any twolight sources of them are included, and the first compatible opticalelement has the first compatible function for forming a converged-lightspot necessary for conducting reproducing and/or recording ofinformation for at least one of the first, second and third opticalinformation recording media, while, the second compatible opticalelement has, in combination with the first compatible optical element,the second compatible function for forming a converged-light spotnecessary for conducting reproducing and/or recording of information forthe other optical information recording medium among the first, secondand third optical information recording media, and is arranged on thecommon optical path for the first, second and third light sources toinclude a diffractive optical element having the first diffractivestructure, and they are structured so that a plurality of ring-shapedzonal optical surfaces with an optical axis as the center are formed onat least one optical surface of at least one optical element among thefirst and second compatible optical elements and the diffractive opticalelement, the plural ring-shaped zonal optical surfaces are formedcontinuously through stepped surfaces, a converged-light spot caused bym^(th) (m represents a natural number) diffracted light generated by thediffractive optical element on the first optical information recordingmedium and a converged-light spot caused by n^(th) (n represents anatural number satisfying n≠m) diffracted light generated by thediffractive optical element on the second optical information recordingmedium.

Item 2-51

The invention described in Item 2-51 is the optical element according toItem 2-50, wherein the first compatible optical element is an objectiveoptical element.

Item 2-52

The invention described in Item 2-52 is the optical element according toItem 2-50 or Item 2-51, wherein the second compatible optical element isa phase difference plate.

Item 2-53

The invention described in Item 2-53 is the optical element according toItem 2-50 or Item 2-51, wherein the second compatible optical element isa liquid crystal element.

Item 2-54

The invention described in Item 2-53 is the optical element according toItem 2-50 or Item 2-51, wherein the second compatible optical element isa diffractive optical element.

Item 2-55

The invention described in Item 2-53 is the optical element according toeither one of Items 2-50-2-54, wherein a light-converging optical systemcomposed of the first compatible optical element, the second compatibleoptical element, the diffractive optical element an objective opticalelement that constitutes an optical pickup device has an optical systemmagnification which is substantially the same for the light fluxeshaving respectively wavelengths λ, λ2 and λ3, and the first and secondcompatible functions correct spherical aberration caused by a wavelengthdifference and that caused by a difference of protective substratethickness between optical information recording media.

Item 2-56

The invention described in Item 2-56 is the optical element according toItem 2-55, wherein the optical system magnification is substantiallyzero.

Item 2-57

The invention described in Item 2-57 is the optical element according toeither one of Items 2-50-2-54, wherein a light-converging optical systemcomposed of the first compatible optical element, the second compatibleoptical element, the diffractive optical element an objective opticalelement that constitutes an optical pickup device has an optical systemmagnifications which are different for the light fluxes havingrespectively wavelengths λ1, λ2 and λ3, and the first and secondcompatible functions correct spherical aberration caused by a wavelengthdifference, spherical aberration caused by a difference of protectivesubstrate thickness between optical information recording media andspherical aberration caused by a difference between optical systemmagnifications of the light-converging optical system.

Item 2-58

The invention described in Item 2-58 is the optical element according toeither one of Items 2-43-2-57, wherein there is provided an opticalcorrecting element for conducting temperature compensation and/orchromatic aberration compensation for at least one of converged-lightspots formed respectively on the first, second and third opticalinformation recording media.

Item 2-59

The invention described in Item 2-58 is the optical element according toItem 2-44 or Item 2-50, wherein NA3<NA1 and NA3<NA1 are satisfied whenNA1 represents a numerical aperture of a converged-light spot formed bythe light flux with wavelength λ1 on the first optical informationrecording medium, NA2 represents a numerical aperture of aconverged-light spot formed by the light flux with wavelength λ2 on thesecond optical information recording medium and NA3 represents anumerical aperture of a converged-light spot formed by the light fluxwith wavelength λ3 on the third optical information recording medium.

Item 2-60

The invention described in Item 2-60 is the optical element according toItem 2-59, wherein a plurality of ring-shaped zonal optical surfaces arerepresented by at least one optical surface of at least oneoptical-element among the first compatible optical element, the secondcompatible optical element and the diffractive optical element, and whena light flux with wavelength λ3 that forms a converged-light spot havingnumerical aperture NA3 on the third optical information recording mediumis formed at an area through which the light flux passes, and when Rsrepresents the ring-shaped zonal optical surface including an opticalaxis among the plural ring-shaped zonal optical surfaces, and R1represents the ring-shaped zonal optical surface which is farthest fromthe optical axis, light fluxes having respectively wavelengths λ1, λ2and λ3 which have passed through the ring-shaped zonal optical surfacesRs are used for reproducing and/or recording for respective opticalinformation recording media, while, a light flux with wavelength λ3 thathas passed through the ring-shaped zonal optical surface R1 is used forreproducing and/or recording for the third optical information recordingmedium.

Item 2-61

The invention described in Item 2-61 is the optical element according toItem 2-60, wherein the first diffractive structure is formed on the areawhich is at least one optical surface of the diffractive optical elementand through which the light flux with the wavelength λ3 which forms alight-converted spot with numerical aperture NA3 on the third opticalinformation recording medium, and a converged-light spot is formed onthe third optical information recording medium by k^(th) (k is a naturalnumber) order diffracted light the light flux with wavelength λ3generated by the first diffractive structure, satisfying k=m/2, 370nm≦λ1≦430 nm and 760 nm≦λ3≦810 nm.

Item 2-62

The invention described in Item 2-62 is the optical element according toItem 2-60 or Item 2-61, wherein each of light fluxes having respectivelywavelengths λ1 and λ2 which have passed through the ring-shaped zonaloptical surface Rs is converged on an information recording surface ofeach optical information recording medium, to be substantially free fromany aberrations.

Item 2-63

The invention described in Item 2-20 is the optical element according toItem 2-61 or Item 2-62, wherein the ring-shaped zonal optical surfaceand the first diffractive structure are formed on the same surface ofthe diffractive optical element.

Item 2-64

The invention described in Item 2-64 is the optical element according toeither one of Items 2-60-2-63, wherein both of the diffractionefficiency of the m^(th) order diffracted light and that of the n^(th)order diffracted light are 80% or more.

Item 2-65

The invention described in Item 2-65 is the optical element according toeither one of Items 2-60-2-64, wherein the diffraction efficiency of thek^(th) order diffracted light is 50% or more.

Item 2-66

The invention described in Item 2-66 is the optical element according toeither one of Items 2-60-2-65, wherein a wave-front aberration of theconverged-light spot formed on the third optical information recordingmedium by the light flux with wavelength λ3 is 0.040 (λ3 rms) or less.

Item 2-67

The invention described in Item 2-67 is the optical element according toeither one of Items 2-60-2-66, wherein paraxial rays of the light fluxwith wavelength λ3 are converged on a point which is closer to the lightsource than the position in the direction of an optical axis which makesa wave-front aberration of the converged-light spot formed on the thirdoptical information recording medium by the light flux with wavelengthλ3 to be minimum.

Item 2-68

The invention described in Item 2-68 is the optical element according toeither one of Items 2-60-2-67, wherein the light fluxes havingrespectively wavelengths λ1 and λ2 enter the diffractive optical elementat the same angle of divergence, or enter as the same infinite light,and the first diffractive structure makes the diffraction effect by adifference between the wavelengths λ1 and λ2 to correct sphericalaberration caused by a difference between the wavelengths λ1 and λ2 andby refraction function of the optical surface on which the firstdiffractive structure is provided and spherical aberration caused by adifference between protective substrate thickness t1 and t2.

Item 2-69

The invention described in Item 2-69 is the optical element according toeither one of Items 2-60-2-68, wherein m=8 and n=5 are satisfied.

Item 2-70

The invention described in Item 2-70 is the optical element according toeither one of Items 2-60-2-68, wherein m=6 and n=4 are satisfied.

Item 2-71

The invention described in Item 2-71 is the optical element according toeither one of Items 2-60-2-68, wherein m=2 and n=1 are satisfied.

Item 2-72

The invention described in Item 2-72 is the optical element according toeither one of Items 2-60-2-71, wherein 1.9×λ1≦λ3≦2.1×λ1 is satisfied.

Item 2-73

The invention-described in Item 2-73 is the optical element according toeither one of Items 2-61-2-72, wherein the light flux with wavelength λ3which has passed the ring-shaped optical surface Rs and the light fluxwith wavelength λ3 which has passed the ring-shaped optical surface R1are converged to be away each other by 10 nm or more in the direction ofan optical axis.

Item 2-74

The invention described in Item 2-74 is the optical element according toItem 2-73, wherein −0.1π≦φ≦0.1π is satisfied by phase difference φ atthe converged-light spots for the light flux with wavelength λ3 whichhas passed through the ring-shaped optical surface Rs and the light fluxwith wavelength λ3 which has passed through the ring-shaped opticalsurface other than the ring-shaped optical surface Rs.

Item 2-75

The invention described in Item 2-75 is the optical element according toItem 2-73 or Item 2-74, wherein a phase difference concerning the lightflux with wavelength λ3 is varied before and after the light flux passesthrough the adjoining ring-shaped zonal optical surfaces.

Item 2-76

The invention described in Item 2-76 is the optical element according toeither one of Items 2-73-2-75, wherein the phase difference of at leastone of the light fluxes respectively with wavelengths λ1 and λ2 is notvaried before and after the light flux passes through the adjoiningring-shaped zonal optical surfaces.

Item 2-77

The invention described in Item 2-77 is the optical element according toeither one of Items 2-73-2-76, wherein the number of the ring-shapedzonal optical surfaces is any number within a range from 2 to 10.

Item 2-78

The invention described in Item 2-78 is the optical element according toeither one of Items 2-61-2-72, wherein the first diffractive structureis formed on an area through which the light fluxes having respectivelywavelengths λ1, λ2 and λ3 which form converged-light spots on respectiveoptical information recording media after passing through thering-shaped zonal optical surface Rs pass, and light-converging positionfB3 of the light flux with wavelength λ3 that forms a converged-lightspot after passing through the ring-shaped zonal optical surface R1satisfies |fB3|≦5 μm in the direction of an optical axis for the bestimage surface for the converged-light spot that is formed by the lightflux with wavelength λ3 on the third optical information recordingmedium.

Item 2-79

The invention described in Item 2-79 is the optical element according toItem 2-78, wherein an area on the optical surface on which the firstdiffractive structure is formed through which the light fluxes havingrespectively wavelengths λ1, λ2 and λ3 which form converged-light spotsrespectively on optical information recording media after passingthrough the ring-shaped zonal optical surface R1 pass is a refractinginterface.

Item 2-80

The invention described in Item 2-80 is the optical element according toItem 2-78, wherein the second diffractive structure is formed on an areaon the optical surface on which the first diffractive structure isformed through which the light fluxes having respectively wavelengthsλ1, λ2 and λ3 which form converged-light spots respectively on opticalinformation recording media after passing through the ring-shaped zonaloptical surface R1 pass.

Item 2-81

The invention described in Item 2-81 is the optical element according toItem 2-80, wherein a combination of diffracted light that generates themaximum diffraction efficiency among diffracted light of respectivelight fluxes generated by the first diffractive structure when lightfluxes having respectively wavelengths λ1, λ2 and λ3 enter is differentfrom a combination of diffracted light that generates the maximumdiffraction efficiency among diffracted light of respective light fluxesgenerated by the second diffractive structure.

Item 2-82

The invention described in Item 2-82 is the optical pickup deviceaccording to Item 2-81, wherein a combination of diffracted light thatgenerates the maximum diffraction efficiency among diffracted light ofrespective light fluxes generated by the second diffractive structurewhen light fluxes having respectively wavelengths λ1, λ2 and λ3 enter isa combination of 1, 1 and 1.

Item 2-83

The invention described in Item 2-83 is the optical element according toeither one of Items 2-78-2-82, wherein the light flux with wavelength λ1which has passed through ring-shaped zonal optical surface R1 isconverged on an information recording surface of the first opticalinformation recording medium, to be substantially free from anyaberrations.

Item 2-84

The invention described in Item 2-84 is the optical element according toeither one of Items 2-78-2-83, wherein the light flux with wavelength λ2which has passed through ring-shaped zonal optical surface R1 isconverged on an information recording surface of the second opticalinformation recording medium, to be substantially free from anyaberrations.

Item 2-85

The invention described in Item 2-42 is the optical element according toeither one of Items 2-78-2-84, wherein a length in the direction that isin parallel with an optical axis of the stepped surface closer to theoptical axis among two stepped surfaces following the ring-shaped zonaloptical surface R1 is shorter than a length in the direction that is inparallel with an optical axis of the other stepped surface.

Item 2-86

The invention described in Item 2-86 is the optical element according toeither one of Items 2-78-2-85, wherein the number of the ring-shapedzonal optical surfaces is two.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an optical pickup device relating to theinvention.

FIG. 2 is a diagram of an optical pickup device in another embodimentrelating to the invention.

FIG. 3 is a diagram of an optical pickup device in still anotherembodiment relating to the invention.

FIG. 4 is a diagram of an optical pickup device in further anotherembodiment relating to the invention.

FIG. 5 is a vertical sectional view of primary portions showing thestructure of an objective optical element.

FIG. 6( a) to 6(c) each shows a vertical spherical aberration diagram.

FIG. 7( a) to 7(c) each shows a vertical spherical aberration diagram.

FIG. 8 is a vertical sectional view of primary portions showing thestructure of an objective optical element.

FIG. 9 is a vertical sectional view of primary portions showing thestructure of an objective optical element.

FIG. 10( a) to 10(c) each shows a vertical spherical aberration diagram.

FIG. 11 is a vertical sectional view of primary portions showing thestructure of an objective optical element.

FIG. 12( a) to (c) each shows a vertical spherical aberration diagram.

FIG. 13 (a) and FIG. 13 (b) each shows a sectional view of an objectivelens.

FIG. 14 shows a sectional view of an objective lens.

FIG. 15 is a graph showing an amount of fluctuation of verticalspherical aberration.

FIG. 16 is a diagram showing wave-front aberration and diffractionefficiency in Example 1.

FIG. 17 shows a graph showing a light-converged position fB and anumerical aperture on AOD in Example 1.

FIG. 18 shows a graph showing a light-converged position fB and anumerical aperture on DVD in Example 1.

FIG. 19 shows a graph showing a light-converged position fB and anumerical aperture on CD in Example 1.

FIG. 20 is a graph showing an amount of fluctuation of verticalspherical aberration.

FIG. 21 is a diagram showing wave-front aberration and diffractionefficiency in Example 2.

FIG. 22 shows a graph showing a light-converged position fB and anumerical aperture on AOD in Example 2.

FIG. 23 shows a graph showing a light-converged position fB and anumerical aperture on DVD in Example 2.

FIG. 24 shows a graph showing a light-converged position fB and anumerical aperture on CD in Example 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings, there will be explained in detail as followsthe contents of the invention, to which, however, embodiments of theinvention are not limited.

First Embodiment

The invention described in Item 1-1 will be explained as follows,referring to FIG. 1.

In the present example, a “high density optical disc” employing theso-called violet laser having a working wavelength of 405 nm is aimed,and there are imagined “high density optical disc” whose protectivesubstrate thickness t1 is 0.6 mm, DVD whose protective substratethickness t2 is 0.6 mm and CD whose protective substrate thickness t3 is1.2 mm, respectively as the first optical information recording medium,the second optical information recording medium and the third opticalinformation recording medium.

FIG. 1 is a schematic diagram showing an optical pickup device relatingto the invention.

Laser diode LD1 is a first light source in which a violet laser whosewavelength λ1 is 405 nm is used, and lasers within a range of 390 nm-420nm in terms of wavelength can be employed according to circumstances.LD2 is a second light source in which a red laser whose wavelength λ2 is655 nm is used, and lasers within a range of 630 nm-680 nm in terms ofwavelength can be employed according to circumstances. LD3 is a thirdlight source in which an infrared laser whose wavelength λ3 is 780 nm isused, and lasers within a range of 750 nm-800 nm in terms of wavelengthcan be employed according to circumstances.

Beam splitter BS1 transmits a beam emitted from LD1 toward OBLrepresenting an objective optical element and also has a function tomake reflected light (returning light) coming from an optical disc (afirst optical information recording medium) to pass through sensor lensgroup SL1 to be converged on light-receiving sensor S1. The function ofBS2 is the same as the foregoing.

BS3 is arranged for the purpose of putting a light flux coming from LD1and a light flux coming from LD2 on the same optical path. BS4 isarranged for the purpose of putting a light flux coming from LD3 and alight flux coming from BS3 on the same optical path.

A light flux emitted from LD1 passes through BS1 and enters collimatorCL1 where the light flux is collimated into the infinite parallel light,and then, passes through BS3 and BS4 to enter objective lens OBLrepresenting an objective optical element. Then, it forms aconverged-light spot on an information recording surface through aprotective substrate of the first optical information recording medium.After being reflected on the information recording surface, it takes thesame path to pass through collimator CL1 and is deflected by BS1 to passthrough lens sensor LS1 to be converged on sensor S1 to be convertedphotoelectrically into electric signals.

A light flux emitted from LD2 also forms a converged-light spot on anoptical disc (a second optical information recording medium) and isreflected to be converged finally on sensor S2. Incidentally, theforegoing also applies to a light flux emitted from LD3, but in thiscase, diffracting plate DP is provided in place of a beam splitter sothat return light may be converged on sensor S3. When conductingreproducing of information from CD, the above-mentioned structure can beused because an amount of received light can be less compared with DVDand “high density optical disc”.

Though objective optical element OBL is a single lens in this diagram,it may also be composed of a plurality of optical elements in case ofneed. Its material may be either plastic resin or glass.

How a light flux emitted from LD 1 and a light flux emitted from LD 2are converged on an information recording surface through respectivelyprotective base plates respectively of optical discs D1 and D2 are drawnon the left side of the optical axis, and how a light flux emitted fromLD3 is converged on an information recording surface through aprotective substrate of optical disc D3 is drawn on the right side ofthe optical axis of OBL. In this way, the basic position is switched byan unillustrated actuator depending on an optical disc to be reproducedand/or recorded, and focusing is conducted from its standard position.

A numerical aperture required on objective optical element OBL variesdepending on a thickness of a protective substrate of each opticalinformation recording medium and on a size of a pit. In this case, thenumerical aperture for CD is 0.45, and that for DVD and a “high densityoptical disc” is 0.65. However, it is possible to select the numericalaperture within a range of 0.43-0.50 for CD and of 0.58-0.68 for DVDaccording to circumstances.

Incidentally, IR represents a diaphragm to cut unwanted light.

In the present example, objective optical element OBL is made to havethe role of “a diffractive optical element that is arranged in a commonoptical path for the first, second and third light sources and has adiffractive structure”. Therefore, a serrated diffractive structure isprovided on the objective optical element.

A pitch of the serration (diffracting power) and its depth (a blazedwavelength) are established so that a light flux coming from the firstlight source is formed as a converged-light spot by the second orderdiffracted light for a “high density optical disc”, and a light fluxcoming from the second light source is formed as a converged-light spotby the first order diffracted light for DVD.

By using light each having a different diffraction order number, it ispossible to enhance diffraction efficiency in each occasion and tosecure an amount of light.

For CD, it is preferable to make a light flux from the third lightsource to be the diffracted light having the order number equivalent tothat for DVD, but the other order number may also be used according tocircumstances. In the present example, a converged-light spot is formedas diffracted light having the first order number equivalent to that forDVD.

Though there has been explained an example wherein a diffractivestructure is provided on an objective optical element as a diffractiveoptical element in the present example, it is also possible to provide adiffractive structure generating such diffracted light with differentorder number on a collimator, or to provide another optical element inan optical path.

Further, for switching of openings, it is possible to apply knowntechnologies including a diffractive optical element.

Incidentally, though there has been explained reproducing of informationin the aforesaid example, basic structures and optical functions remainunchanged even for recording of information, and thermochemical changesare made in a recording layer for recording by forming a converged-lightspot on a recording surface of an optical information recording medium.

Further, it is naturally possible to provide an optical element havingan optical correcting structure for conducting temperature compensationand/or chromatic aberration compensation in an optical path in case ofneed. These optical correcting structures can be realized by adiffractive structure or by a phase difference furnishing structure, andcan be provided on an objective optical element, a collimator and otherelements.

Second Embodiment

The invention described in Item 1-5 will be explained as follows,referring to FIG. 1.

For each optical element, an explanation will be omitted for thefunctions which are the same as those in the First Embodiment.

In the present example, objective optical element OBL is made to havethe role of the first compatible optical element. Collimator CL3 is madeto have the role of the second compatible optical element.

In other words, the objective optical element OBL representing the firstcompatible optical element is arranged in the optical path through whichall light sources pass, and the collimator CL3 representing the secondcompatible optical element is arranged in the optical path through whichthe third light source only passes.

By the way, the objective optical element OBL representing the firstcompatible optical element has a diffractive structure by whichcompatibility (first compatibility function) between “high densityoptical disc” and DVD is attained.

Specifically, spherical aberration based on a wavelength differencebetween the first light source and the second light source is corrected.Further, it is also possible to obtain the same optical function with aphase difference furnishing structure in place of the diffractivestructure.

Incidentally, with respect to an optical information recording medium,if there is a difference in protective substrate thickness, there iscaused spherical aberration based on the difference. However, in thiscase, the protective substrate having a thickness of 0.6 mm is used forboth “high density optical disc” and DVD, which prevents occurrence ofspherical aberration based on a difference in protective substratethickness.

The diffractive structure is provided also on the collimator CL3representing the second compatible optical element. When thisdiffractive structure is combined with the diffractive structure on theobjective optical element OBL, compatibility between “high densityoptical disc” and CD and compatibility (second compatibility function)between DVD and CD are attained.

To be concrete, when compatibility between “high density optical disc”and CD is considered, working wavelengths and protective substratethicknesses are different, and therefore, both spherical aberrationbased on a difference in thicknesses between the first light source andthe third light source and spherical aberration based on a difference ofprotective substrate thicknesses 19 (0.1 mm and 1.2 mm) are corrected.

The foregoing also applies to compatibility between DVD and CD, andspherical aberration based on a difference in wavelengths between thesecond light source and the third light source and spherical aberrationbased on a difference in protective substrate thicknesses 19 (0.6 mm and1.2 mm) are corrected.

Owing to the foregoing, it is possible to form a satisfactoryconverged-light spot on each optical information recording medium.

Since a converged-light spot can be formed by diffracted light with adifferent diffraction order number, in the same way as in the previousembodiment, an amount of light can be secured for “high density opticaldisc” and DVD, and recording and/or reproducing of information can beconducted.

Though there has been shown an example (Item 1-9) wherein a diffractiveoptical element is provided on collimator CL3 as the second compatibleoptical element, it is also possible to obtain the same optical functioneven when a dichroic filter having a property of selecting a wavelengthand a liquid crystal capable of switching optical functionselectrically, for example, are used (Items 1-7 and 1-8). In particular,a liquid crystal makes dynamic control possible because it has afunction to change the refractive index.

In addition, for the first compatible function and the second compatiblefunction, it is possible to attain them by providing a phase differencefurnishing structure.

Third Embodiment

This example is one corresponding to the invention of Item 1-11, and itis an optical pickup device wherein a coupling lens is provided in placeof the prescribed collimator for the structure shown in FIG. 1.Specifically, coupling lenses Co 1-Co 3 are provided in place of thecollimators CL 1-3.

Since there is not provided a collimator that collimates incident lightfrom a light source into parallel light, finite divergent light entersan objective optical element. Since the power of the coupling lens isnot as great as that of the collimator, the coupling lens is small insize, and this structure using the coupling lens makes a pickup deviceto be small.

When a finite divergent light is used in place of an infinite parallellight, magnification of a light flux entering objective optical elementOBL is changed, and thereby, spherical aberration based on a wavelengthdifference and that based on a substrate thickness difference arecorrected, which has been known. However, there is sometimes an occasionwhere sufficient correction is impossible.

Further, the use of a finite light causes a problem that temperaturecharacteristics are deteriorated, and also causes spherical aberrationbased on a difference of magnification of an incident light flux, whichneeds to be solved.

In the present example, therefore, a light flux having a magnificationwhich is different dependent on each wavelength is made to enterobjective optical element OBL, and spherical aberration based on awavelength difference, that based on a protective substrate thicknessdifference and that based on a magnification difference of a light fluxare corrected by the first and second compatible optical elements.

The first compatible optical element is one wherein a diffractiveoptical element is proved on an objective optical element in the sameway as in the Second Embodiment, while, the second compatible opticalelement is one wherein a diffractive optical element is proved oncoupling lens Co 3.

Due to this, all of the light fluxes emitted respectively from thefirst-third light sources enter objective optical element OBL in theform of a finite divergent light, thus, all spherical aberrations arecorrected and satisfactory converged-light spots are formed.

In this case, an infinite divergent light enters objective opticalelement OBL for all light sources. However, it is also possible toarrange so that an infinite divergent light enters objective opticalelement OBL for only one light source, and an infinite parallel lightenters for other light sources.

Fourth Embodiment

Another embodiment of the invention of Item 1-1 will be explained asfollows, referring to FIG. 2. Those having the same symbols as those inthe First Embodiment have the same functions basically, thus, differentitems only will be explained. Incidentally, the same also applies to theoptical functions.

In this example, the light source is composed of two units. To beconcrete, LD2′ in FIG. 2 represents a light source unit of the so-calledtwo-laser one package wherein the second light source (light source forDVD) and the third light source (light source for CD) are housed in thesame package.

Since an adjustment is made so that the second light source in thepackage may be on the optical axis, the third light source is positionedto be away from the optical axis, and an image height is caused.However, technologies to improve this characteristic have already beenknown, and these technologies can be applied according to circumstances.In this case, correction plate DP is used to conduct that correction. Onthe correction plate DP, there is formed a grating which corrects thedeviation from the optical axis and contributes to light-converging onsensor S2.

Incidentally, solid lines drawn from LD2′ represent a light flux for DVDand dotted lines drawn from LD2′ represent a light flux for CD.

BS2 is arranged for putting a light flux coming from LD1 and a lightflux coming from LD2′ on the same optical path. BS3 is arranged to makea light flux coming from LD2′ to enter sensor lens S12.

A light flux emitted from LD1 passes through BS1 and enters collimatorCL1 which collimates the light flux to be infinite parallel light, then,passes through BS2 to enter objective lens OBL representing an objectiveoptical element. Then, it forms a converged-light spot on an informationrecording surface through a protective substrate of the first opticalinformation recording medium. After being reflected on the informationrecording surface, it takes the same path and passes through collimatorCL1 to be converged on sensor S1 by BS1 through sensor lens SL1, to beconverted photoelectrically to become electric signals.

A light spot emitted from LD2′ also forms a converged-light spot on anoptical disc (second optical information recording medium or thirdoptical information recording medium) equally and is reflected to formfinally on sensor s2.

In the present example, objective optical element OBL is made to have arole of “a diffractive optical element which is arranged in a commonoptical path for the first, second and third optical informationrecording media and has a diffractive structure”. Therefore, a serrateddiffractive structure is provided on an objective optical element.

A pitch of the serration (diffracting power) and its depth (a blazedwavelength) are established so that a light flux coming from the firstlight source is formed as a converged-light spot by the second orderdiffracted light for a “high density optical disc”, and a light fluxcoming from the second light source is formed as a converged-light spotby the first order diffracted light for DVD.

By using light each having a different diffraction order number, it ispossible to enhance diffraction efficiency in each occasion and tosecure an amount of light.

For CD, it is preferable to make a light flux from the third lightsource to be the diffracted light having the order number equivalent tothat for DVD, but the other order number may also be used according tocircumstances. In the present example, a converged-light spot is formedas diffracted light having the first order number equivalent to that forDVD.

Though there has been explained an example wherein a diffractivestructure is provided on an objective optical element as a diffractiveoptical element in Items 1-3 and 1-4, it is also possible to provide adiffractive structure generating such diffracted light with differentorder number on a collimator CL1, or to provide another optical elementin an optical path.

Further, for switching of openings, it is possible to apply knowntechnologies including a diffractive optical element.

Incidentally, though there has been explained reproducing of informationin the aforesaid example, basic structures and optical functions remainunchanged even for recording of information, and thermochemical changesare made in a recording layer for recording by forming a converged-lightspot on a recording surface of an optical information recording medium.

Further, it is naturally possible to provide an optical element havingan optical correcting structure for conducting temperature compensationand/or chromatic aberration compensation in an optical path in case ofneed. These optical correcting structures can be realized by adiffractive structure or by a phase difference furnishing structure, andcan be provided on an objective optical element, a collimator and otherelements.

Fifth Embodiment

Another embodiment of the invention described in Item 1-5 will beexplained as follows, referring to FIG. 2.

For each optical element, an explanation will be omitted for thefunctions which are the same as those in the Fourth Embodiment.

In the present example, objective optical element OBL is made to havethe role of the first compatible optical element. Collimator CL2 is madeto have the role of the second compatible optical element.

In other words, the objective optical element OBL representing the firstcompatible optical element is arranged in the optical path through whichall light sources pass, and the collimator CL2 representing the secondcompatible optical element is arranged in the optical path through whichthe second light source and the third light source pass.

By the way, the objective optical element OBL representing the firstcompatible optical element has a diffractive structure which contributesto formation of a converged-light spot necessary for a “high densityoptical disk”.

Specifically, spherical aberration based on a wavelength differencebetween the first light source and the second light source and sphericalaberration based on a wavelength difference between the first lightsource and the third light source are corrected. Further, sphericalaberration based on a protective substrate thickness difference betweenthe “high density optical disk” and CD is corrected.

Further, it is also possible to obtain the same optical function with aphase difference furnishing structure in place of the diffractivestructure.

Incidentally, with respect to an optical information recording medium,if there is a difference in protective substrate thickness, there iscaused spherical aberration based on the difference. However, in thiscase, the protective substrate having a thickness of 0.6 mm is used forboth “high density optical disc” and DVD, which prevents occurrence ofspherical aberration based on a difference in protective substratethickness.

The diffractive structure is provided also on the collimator CL2representing the second compatible optical element. When thisdiffractive structure is combined with the diffractive structure on theobjective optical element OBL, compatibility (second compatibilityfunction) between DVD and is attained.

When compatibility between DVD and CD is considered, working wavelengthsand protective substrate thicknesses are different, and therefore, bothspherical aberration based on a difference in thicknesses between thesecond light source and the third light source and spherical aberrationbased on a difference of protective substrate thicknesses 19 (0.6 mm and1.2 mm) are corrected.

Owing to the foregoing, it is possible to form a satisfactoryconverged-light spot on each optical information recording medium.

Since a converged-light spot can be formed by diffracted light with adifferent diffraction order number, in the same way as in the previousembodiment, an amount of light can be secured for “high density opticaldisc” and DVD, and recording and/or reproducing of information can beconducted.

Though there has been shown an example (Item 1-9) wherein a diffractiveoptical element is provided on collimator CL2 as the second compatibleoptical element, it is also possible to obtain the same optical functioneven when a dichroic filter having a property of selecting a wavelengthand a liquid crystal capable of switching optical functionselectrically, for example, are used (Items 1-7 and 1-8). In particular,a liquid crystal makes dynamic control possible because it has afunction to change the refractive index.

In addition, for the first compatible function and the second compatiblefunction, it is possible to attain them by providing a phase differencefurnishing structure.

Sixth Embodiment

This example is another example corresponding to the invention of Item1-11, and it is an optical pickup device wherein a coupling lens isprovided in place of the prescribed collimator for the structure shownin FIG. 2. Specifically, coupling lenses Co 1 and Co 2 are provided inplace of the collimators CL 1 and Cl 2.

Since there is not provided a collimator that collimates incident lightfrom a light source into parallel light, finite divergent light entersan objective optical element. Since the power of the coupling lens isnot as great as that of the collimator, the coupling lens is small insize, and this structure using the coupling lens makes a pickup deviceto be small.

When a finite divergent light is used in place of an infinite parallellight, magnification of a light flux entering objective optical elementOBL is changed, and thereby, spherical aberration based on a wavelengthdifference and that based on a substrate thickness difference arecorrected, which has been known. However, there is sometimes an occasionwhere sufficient correction is impossible.

Further, the use of a finite light causes a problem that temperaturecharacteristics are deteriorated, and also causes spherical aberrationbased on a difference of magnification of an incident light flux, whichneeds to be solved.

In the present example, therefore, a light flux having a magnificationwhich is different dependent on each wavelength is made to enterobjective optical element OBL, and spherical aberration based on awavelength difference, that based on a protective substrate thicknessdifference and that based on a magnification difference of a light fluxare corrected by the first and second compatible optical elements.

The first compatible optical element is one wherein a diffractiveoptical element is proved on an objective optical element in the sameway as in the Fifth Embodiment, while, the second compatible opticalelement is one wherein a diffractive optical element is proved oncoupling lens Co 2.

Due to this, all of the light fluxes emitted respectively from thefirst-third light sources enter objective optical element OBL in theform of a finite divergent light, thus, all spherical aberrations arecorrected and satisfactory converged-light spots are formed.

In this case, an infinite divergent light enters objective opticalelement OBL for all light sources. However, it is also possible toarrange so that an infinite divergent light enters objective opticalelement OBL for only one light source, and an infinite parallel lightenters for other light sources.

Sixth Embodiment

Inventions in Item 1-16 or 17 will be explained as follows, referring toFIG. 3.

For each optical element, an explanation will be omitted for thefunctions which are the same as those in the First-Fifth Embodiments.

Single plastic lens OBL representing an objective optical element isunited with liquid crystal element LCD solidly by lens holder LH. ACrepresents an actuator which can hold the lens holder LH to move in thedirection of an optical axis for focusing.

On the optical functional surface of the single plastic lens OBL, thereis provided a diffractive structure which corresponds to the diffractiveoptical element that is mentioned in the Items.

Though the single lens made of plastic is mentioned in this case, it mayalso be one wherein plural lenses of two or more are combined, or it maybe a lens made of glass.

On the liquid crystal element LCD, there is provided a pattern that issymmetrical about an optical axis, and it is possible to switch opticalfunctions for an incident light flux by changing electric state ofenergizing. This corresponds to the compatible optical element mentionedin the Items.

A structure is arranged so that a light flux that is made by eachcollimator to be an infinite parallel light may enter the liquid crystalelement.

Though an optical system utilizing an infinite parallel light hasvarious merits, it also has a demerit, on the contrary, that low orderaberrations having an influence on formation of a converged-light spotare caused. It is therefore difficult to cope with three types ofoptical information recording media by means of an objective opticalelement alone.

In the invention described in Item 1-16, therefore, a diffractivestructure and an aspheric surface are designed so that an objectiveoptical element provided with a diffracting surface may be usedindependently for a “high density optical disc” representing the firstoptical information recording medium, but it may have insufficientfunctions (functions unable to form a sufficient converged-light spotfor reproducing and/or recording of information).

Then, a compatible optical element (liquid crystal element) that isseparate from the foregoing is prepared to be combined with an objectiveoptical element, and insufficient points in the objective opticalelement are improved so that sufficient converged-light spot may beformed for reproducing and/or recording of information for the secondoptical information recording medium (DVD) and the third opticalinformation recording medium (CD) may be formed.

Incidentally, for a light flux with wavelength λ2 (655 nm) used for thesecond optical information recording medium (DVD) and a light flux withwavelength λ3 (780 nm) used for the third optical information recordingmedium (CD), the objective optical element uses a diffracted light withorder number that is different from that of a light flux with wavelengthλ1 (405 nm) so that the diffraction efficiency may be high to theutmost, and a load for correction by a liquid crystal element may bereduced, although the efficiency is insufficient.

In the invention described in Item 1-17, design of an objective opticalelement equipped with a diffracting surface is devised, and adiffractive structure and an aspheric surface are designed so that theobjective optical element may be used independently for two types ofoptical information recording media including the first opticalinformation recording medium (high density optical disc) and either oneof the second optical information recording medium (DVD) and the thirdoptical information recording medium (CD). An optical function ofanother compatible optical element (liquid crystal element) issuperimposed on an optical function of the objective optical element, sothat a sufficient converged-light spot may be formed for reproducingand/or recording of information, for the remaining one type of opticalinformation recording medium.

As stated above, the compatible optical element representing a liquidcrystal element does not generate optical functions for the light fluxwith wavelength λ1, and generates optical functions for the light fluxwith wavelength λ2 and/or the light flux with wavelength λ3 as occasiondemands.

Incidentally, the objective optical element and the liquid crystalelement both held solidly by lens holder LH can be regarded together asan objective optical element, and the present embodiment may beexpressed as an applied example of the invention described in Item 1-1.

As the diffractive structure which has been described, it is alsopossible to employ a diffractive structure with the so-calledmulti-level structure wherein a shape of a staircase with a prescribednumber of steps is repeated periodically, in addition to the serrateddiffractive structure.

With respect to the order number of the diffracted light generated bythe diffractive structure, various combinations can be used as occasiondemands.

In the case of the invention described in Item 1-16, it is preferable toselect combinations of the following order numbers. Incidentally, krepresents the order number of the diffracted light generated for thelight flux with wavelength λ1, m represents the order number of thediffracted light generated for the light flux with wavelength λ2 and nrepresents the order number of the diffracted light generated for thelight flux with wavelength λ3.

a) k=1, m=0, n=2

b) k=2, m=1, n=1

c) k=2, m=1, n=0

d) k=2, m=2, n=1

e) k=3, m=2, n=2

f) k=4, m=3, n=2

g) k=5, m=3, n=2

h) k=5, m=3, n=3

i) k=6, m=4, n=3

j) k=7, m=4, n=4

k) k=8, m=5, n=4

Combinations of the order numbers identical to the foregoing arepreferable, even when conducting compatibility between the first opticalinformation recording medium and the second optical informationrecording medium with the objective optical element, and attainingcompatibility with the third optical information recording medium bycombining with a liquid crystal element, in the case of inventiondescribed in Item 1-17.

When conducting compatibility between the first optical informationrecording medium and the second optical information recording mediumwith the objective optical element, and attaining compatibility with thethird optical information recording medium by combining with a liquidcrystal element, in the case of invention described in Item 1-17,wavelength of λ1 and that of λ3 show the relationship of almost twotimes, and therefore, if k:n is 2:1, diffracting functions generated areTherefore, it is preferable to employ the following combinations whichare the results of excluding the occasions where k:n is 2:1 from thecombinations from a) to k).

a) k=1, m=0, n=2

c) k=2, m=1, n=0

e) k=3, m=2, n=2

g) k=5, m=3, n=2

h) k=5, m=3, n=3

j) k=7, m=4, n=4

Seventh Embodiment

Another embodiment of the inventions in Item 1-16 or 1-17 will beexplained as follows, referring to FIG. 4.

This is one wherein beam expander BE that is movable in the axialdirection is provided in place of liquid crystal element LCD.

This beam expander is provided with its own actuator to be capable ofmoving in the axial direction. This corrects spherical aberration.

When it is actually used, it is made to advance or retreat in the axialdirection to correct spherical aberration in accordance with a lightflux from the working light source, and an excellent converged-lightspot is formed on the corresponding optical information recordingmedium.

With respect to the objective lens representing a diffractive opticalelement and its functions, they are the same as those in the SixthEmbodiment, and they can form an excellent converged-light spotindependently on at least the first optical information recordingmedium. For the second and third optical information recording media,sufficient power to form a converged-light spot is not possessed, or, aconverged-light spot can be formed on only one of them.

Further, for that purpose, a structure is arranged so that diffractedlight each having a different order number for each wavelength aregenerated.

The beam expander representing a compatible optical element can advanceor retreat in the axial direction as stated above, and thereby,spherical aberration can be corrected. Therefore, by offsettinginsufficient points with an objective optical element, it is possible toform excellent converged-light spots on the second optical informationrecording medium and/or the third optical information recording medium.

Incidentally, there may also be the structure to unite the beam expanderand the objective optical element solidly, although this leads to alarge-sized optical element. This structure is preferable from theviewpoint of aberration correction for the third optical informationrecording medium.

As stated above, the invention makes it possible to realize an opticalpickup device that is compatible for three formats of optical discs andis compact in structure, wherein an amount of light can be secured andperformance is excellent.

Eighth Embodiment

The invention described in Item 2-1 will be explained as follows;referring to FIG. 1 which is explained before.

In this embodiment, light fluxes having respectively wavelengths λ1-λ3emitted respectively from LD1-LD3 enter objective optical element OBLrepresenting a diffractive optical element having the first diffractivestructure as infinite parallel light, in other words, at substantiallythe same angle, as stated above.

Incidentally, “the same angle” means the same angle of divergence or thesame angle of convergence, and in the case of infinite parallel light,an angle of divergence (or an angle of convergence) is zero.

In this embodiment, by using light wherein diffraction order isdifferent based on relationship between wavelength λ1 and wavelength λ2as stated above, it is possible to enhance diffraction efficiency ineach case and to secure an amount of light.

For CD, it is preferable to use k^(th) order (m/2, when the diffractionorder for wavelength λ1 is m) based on relationship between wavelengthλ1 and wavelength λ3.

In this example, a converged-light spot is formed as the first orderdiffracted light, which is the same as one in DVD.

Though there has been explained an example wherein a diffractivestructure is provided on an objective optical element as a diffractiveoptical element, likewise Items 2-4 and 2-6, it is possible to providethis diffractive structure that generates a different order diffractedlight on a collimator, and to provide another optical element in anoptical path.

Ninth Embodiment

The invention described in Item 2-7 will be explained as follows,referring to the FIG. 1.

Though there has been shown an example (Item 2-11) wherein thediffractive optical element is provided on the collimator CL3, as thesecond compatible optical element, in the present example, it is alsopossible to obtain the same optical functions (Item 2-10) by using anoptical element (Item 2-9) wherein an optical path difference furnishingstructure that gives only a phase, or by using a liquid crystal elementcapable of switching optical functions electrically. In particular, theliquid crystal element makes it possible to conduct dynamic controlbecause it has a function to change the refractive index. Further, it isalso possible to conduct compatibility between the first compatibleoptical element and the diffractive structure, and to use a dichroicfilter that plays a role of a CD-side diaphragm only.

In addition to the foregoing, it is also possible to attain by providinga phase difference furnishing structure together with the firstcompatible function and the second compatible function.

Tenth Embodiment

This embodiment corresponds to the invention described in each of Items2-1 and 2-2, and it is an optical pickup device on which a coupling lensis provided in place of a prescribed collimator in the structure shownin FIG. 1. Specifically, coupling lenses Co 1-3 (not shown) are providedin place of collimators CL1-CL3.

Due to this, all light fluxes emitted respectively from the first-thirdlight sources enter the objective optical element OBL in a form of afinite divergent light, thus, all spherical aberrations are corrected,and an appropriate converged-light spot is formed.

Since the coupling lenses Co 1-3 are used in place of the collimators CL1-3 in this case, all light fluxes emitted from all light sources enterthe objective optical element in a form of divergent light. However, itis also possible to make any one of them to be a collimator so thatinfinite parallel light may enter an objective lens.

Eleventh Embodiment

Another embodiment of the invention described in Item 2-1 will beexplained as follows, referring to FIG. 2. Those having the same symbolsas in the First Embodiment have basically the same functions as in theEighth Embodiment, and different ones only will be explainedaccordingly. For the optical functions, the foregoing can substantiallybe applied.

In this embodiment, by using light wherein diffraction order isdifferent based on relationship between wavelength λ1 and wavelength λ2as stated above, it is possible to enhance diffraction efficiency ineach case and to secure an amount of light.

For CD, it is preferable to use k^(th) order (m/2, when the diffractionorder for wavelength λ1 is m) based on relationship between wavelengthλ1 and wavelength λ3.

In this example, a converged-light spot is formed as the first orderdiffracted light, which is the same as one in DVD.

Though there has been explained an example wherein a diffractivestructure is provided on an objective optical element as a diffractiveoptical element, likewise Items 2-4 and 2-6, it is possible to providethis diffractive structure that generates a different order diffractedlight on a collimator CL1, and to provide another optical element in anoptical path.

Twelfth Embodiment

Another embodiment of the invention described in Item 2-7 will beexplained as follows, referring to the FIG. 2. With respect to eachoptical element, an explanation of the functions which are the same asthose in the Eleventh Embodiment will be omitted. In the presentexample, objective optical element OBL is made to have a role of thefirst compatible optical element. On the other hand, collimator CL2 ismade to have a role of the second compatible optical element. Namely,the objective optical element OBL representing the first compatibleoptical element is arranged in the optical path through which all lightsources pass, and the collimator CL2 representing the second compatibleoptical element is arranged in the optical path through which the secondlight source the third light source pass.

Though there has been shown an example (Item 2-11) wherein thediffractive optical element is provided on the collimator CL2, as thesecond compatible optical element, in the present example, it is alsopossible to obtain the same optical functions (Item 2-10) by using anoptical element (Item 2-9) wherein an optical path difference furnishingstructure that gives only a phase, or by using a liquid crystal elementcapable of switching optical functions electrically. In particular, theliquid crystal element makes it possible to conduct dynamic controlbecause it has a function to change the refractive index. Further, it isalso possible to conduct compatibility between the first compatibleoptical element and the diffractive structure, and to use a dichroicfilter that plays a role of a CD-side diaphragm only.

Thirteenth Embodiment

This embodiment corresponds to the invention described in each of Items2-1 and 2-2, and it is an optical pickup device on which a coupling lensis provided in place of a prescribed collimator in the structure shownin FIG. 1. Specifically, coupling lenses Co 1-3 (not shown) are providedin place of collimators CL1-CL3.

Since there is not provided a collimator that collimates an incidentlight coming from a light source to be parallel light, finite divergentlight enters an objective optical element. Since the coupling lens doesnot have power that is as great as that of the collimator, a size of thecoupling lens is small, which makes a pickup device to be small.

Since the coupling lenses Co 1-3 are used in place of the collimators CL1-3 in this case, all light fluxes emitted from all light sources enterthe objective optical element in a form of divergent light. However, itis also possible to make any one of them to be a collimator so thatinfinite parallel light may enter an objective lens.

Fourteenth Embodiment

In this example, as shown in FIG. 5, plural (two) ring-shaped zonaloptical surfaces (Rs and R1) having their centers on optical axis L areformed continuously through stepped surface 20 on an area on opticalsurface 11 (emergence surface) of objective optical element 10 throughwhich a light flux with wavelength λ3 that forms a converged-light spotwith numerical aperture NA3 on an image recording surface of CDrepresenting the third optical information recording medium passes, asshown in FIG. 5. Incidentally, in the following explanation, the totaloptical surface on which ring-shaped zonal optical surfaces are formedis sometimes expressed as “S1 surface”.

It is preferable that the number of these ring-shaped zonal opticalsurfaces is either one within a range of 2-10.

Now, let it be assumed that Rs represents a ring-shaped zonal opticalsurface including optical axis L and R1 represents a ring-shaped zonaloptical surface that is farthest from the optical axis, among the tworing-shaped zonal optical surfaces.

Incidentally, with respect to the ring-shaped zonal optical surface Rsincluding optical axis L, it is assumed to include also an occasionwherein a form of the ring-shaped zonal optical surface viewed in thedirection of optical axis L is not a “ring-shaped zone” but is mostly acircular form whose center is optical axis L. A form of the ring-shapedzonal optical surface Rs in the present embodiment is mostly a circularform when it is viewed in the direction of optical axis L.

It is preferable that a length of the stepped surface 20 closer to theoptical axis L in the direction that is in parallel with optical axis L,among two stepped surfaces 20 each adjoining the ring-shaped zonaloptical surface R1, is shorter than a length of the other steppedsurface 20 in the direction being in parallel with optical axis L.

The ring-shaped zonal optical surface Rs is composed of a refractinginterface, and light fluxes having respectively wavelengths λ1, λ2 andλ3 which have passed the ring-shaped zonal optical surface Rs arearranged to form converged-light spots respectively on informationrecording surfaces of respective optical information recording media(“high density optical disk”, DVD and CD).

The ring-shaped zonal optical surface R1 is also composed of arefracting interface, and it is formed to be deviated toward a lightsource from the ring-shaped zonal optical surface Rs by a prescribeddistance.

A light flux having wavelength λ3 which has passed through thering-shaped zonal optical surface R1 is also arranged to form aconverged-light spot on the information recording surface of the thirdoptical information recording medium.

On the other optical surface 12 (plane of incidence) of objectiveoptical element 10, there is formed a diffracting ring-shaped zonesrepresenting the first diffractive structure 30. Incidentally, in thefollowing explanation, the total optical surface on which the firstdiffractive structure 30 is formed is sometimes expressed as “S2surface”.

The first diffractive structure 30 is formed on an area on plane ofincidence 12 (hereinafter referred to also as area A1) through which alight flux having wavelength λ3 that forms a converged-light spot withnumerical aperture NA3 on the third optical information recording mediumpasses.

The area A1 on which the first diffractive structure is formedcorresponds to an area through which the light fluxes respectivelyhaving wavelengths λ1, λ2 and λ3 which form converged-light spotsrespectively on optical information recording media after passingthrough the ring-shaped zonal optical surface Rs pass.

In the present embodiment, a diffracting ring-shaped zone representingthe second diffractive structure 40 is formed on an area (hereinafterreferred to also as area A2) which is a portion that is farther fromoptical axis L than the area A1 is, and a portion through which thelight fluxes having respectively wavelengths λ1, λ2 and λ3 which formconverged-light spots respectively on optical information recordingmedia pass.

A structure of the area that is farther from the optical axis than thearea A2 is not limited, and diffracting ring-shaped zones are formed inthe present embodiment. An explanation of the diffracting ring-shapedzones will be omitted because they are known.

Thus, m^(th), n^(th) and k^(th) order diffracted light havingrespectively wavelengths λ1, λ2 and λ3 which are generated by the firstdiffractive structure are converged respectively on informationrecording surfaces of optical information recording media to conductreproducing and/or recording of information.

In this case, it is preferable to construct so that light fluxes havingrespectively wavelengths λ1 and λ2 enter objective optical element 10 atthe same angle of divergence or as the same infinite light, and firstdiffractive structure 30 corrects spherical aberration caused when lightfluxes having respectively wavelengths λ1 and λ2 pass respectivelythrough optical surfaces 10 and 12 of the objective optical element 10,by means of a difference between wavelength λ1 and wavelength λ2.

Further, it is preferable to make each diffracted light to be either oneof combinations of (m, n) (8,5), (6,4), (2,1), (5,3), (2,2), (3,2),(10,6), furthermore, it is preferable to make each diffracted light tobe either one of combinations of (m, n, k)=(2,1,1), (2,1,0), (5,3,2),(2,2,1), (3,2,2).

Further, when light fluxes having respectively wavelengths λ1, λ2 and λ3enter, it is preferable that a combination of diffracted light thatcreates the maximum diffraction efficiency among diffracted light ofeach light flux generated by the first diffractive structure 30 isdifferent from a combination of diffracted light that creates themaximum diffraction efficiency among diffracted light of each light fluxgenerated by the second diffractive structure 40.

Although it is preferable that a combination of diffracted light thatgenerates the maximum diffraction efficiency among diffracted light ofrespective light fluxes generated by the second diffractive structure 40is a combination of 1, 1 and 1, the invention is not limited to this.

Further, it is preferable to satisfy k=m/2 under the conditions of 370nm≦λ1≦430 nm and 760 nm≦λ3≦810 nm.

It is further preferable that both of the diffraction efficiency for them^(th) order diffracted light and that for the n^(th) order diffractedlight are 80% or more.

Further, it is preferable that the diffraction efficiency for the k^(th)order diffracted light is 40% or more.

FIG. 6 shows an example of a vertical spherical aberration diagram oneach information recording surface of a high density optical disc, DVDand CD in the case of using objective optical element 10 structured inthe aforesaid way for an optical pickup device. Incidentally, in thefollowing FIGS. 6, 7, 10 and 12, a vertical axis represents a numericalaperture and a horizontal axis represents an amount of sphericalaberration.

As shown in FIG. 6 (a), with respect to a light flux having wavelengthλ1 used for a high density optical disk, its spherical aberration is notchanged within a numerical aperture corresponding to the place wherering-shaped zonal optical surface Rs is formed, namely, a light fluxhaving wavelength λ1 which has passed the ring-shaped zonal opticalsurface Rs is converged on an information recording surface of the firstoptical information recording medium, to be substantially free from anyaberrations.

On the other hand, within a numerical aperture corresponding to theplace where ring-shaped zonal optical surface R1 is formed, sphericalaberrations become discontinuous toward the “under” side.

Incidentally, when the combination of the orders of diffracted lightthat makes the diffraction efficiency to be maximum when light fluxeshaving respectively wavelengths λ1, λ2 and λ3 enter is changed bychanging a form of the second diffraction structure, it is also possibleto make the spherical aberration to be discontinuous toward the “over”side.

It is relatively easy to design forms of S1 surface and S2 surface sothat spherical aberrations may be within a range of no trouble inpractical use when the total area corresponding to numerical apertureNA1 is considered.

As shown in FIG. 6 (b), with respect to a light flux having wavelengthλ2 used for DVD, its spherical aberration is not changed within anumerical aperture corresponding to the place where ring-shaped zonaloptical surface Rs is formed, namely, a light flux having wavelength λ2which has passed the ring-shaped zonal optical surface Rs is convergedon an information recording surface of the second optical informationrecording medium, to be substantially free from any aberrations.

Further, the spherical aberration is not changed even within a numericalaperture corresponding to the place where ring-shaped zonal opticalsurface R1 is formed, namely, a light flux having wavelength λ2 whichhas passed the ring-shaped zonal optical surface R1 is converged on aninformation recording surface of the second optical informationrecording medium, to be substantially free from any aberrations.

Therefore, it is possible to make spherical aberrations to be almostzero when the total area corresponding to numerical aperture NA2 isconsidered.

As shown in FIG. 6 (c), with respect to a light flux having wavelengthλ3 used for CD, its spherical aberration grows greater toward the “over”side within a numerical aperture corresponding to the place wherering-shaped zonal optical surface Rs is formed.

On the other hand, within a numerical aperture corresponding to theplace where ring-shaped zonal optical surface R1 is formed, sphericalaberrations become discontinuous toward the “under” side.

It is relatively easy to design forms of S1 surface and S2 surface sothat spherical aberrations may be within a range of no trouble inpractical use when the total area corresponding to numerical apertureNA3 is considered. Incidentally, a light flux having wavelength λ3 whichhas passed the position which is farther from the optical axis than theplace where the ring-shaped zonal optical surface R1 is formed is,becomes a flare, and a diameter of the spot becomes to be equivalent tothe necessary numerical aperture.

Incidentally, it is preferable that wavefront aberration of aconverged-light spot that is formed by a light flux having wavelength λ3on the third optical information recording medium is 0.050 (λ3 rms) orless.

It is further preferable that a paraxial ray of a light flux havingwavelength λ3 is converged at a position that is closer to the lightsource than the position (best image surface position) in the directionof an optical axis that makes wavefront aberration of theconverged-light spot formed by a light flux having wavelength λ3 on thethird optical information recording medium to be smallest is.

Further, it is preferable that the light flux having wavelength havingwavelength λ3 that has passed through the ring-shaped zonal opticalsurface Rs and the light flux having wavelength having wavelength λ3that has passed through the ring-shaped zonal optical surface R1 areconverged to be away from each other by 10 μm or more.

Further, it is preferable that a light flux of the wavelength λ3 havingpassed through the ring-shaped zone optical surface Rs and a light fluxof the wavelength λ3 having passed through the ring-shaped zone opticalsurface R1 are converged distantly in the optical axis by 5 μm or more.

Further, it is preferable that a length D in the optical axis of astepped surface between neighboring two ring-shaped zonal opticalsurfaces satisfies the formulas:1.5 μm≦D≦2.0 μm,2.0 μm≦D≦3.0 μm,or 3.0 μm≦D≦4.5 μm,

It is preferable that phase difference φ between a light flux withwavelength λ3 that has passed the ring-shaped zonal optical surface Rsand a light flux with wavelength λ3 that has passed a ring-shaped zonaloptical surface other than the ring-shaped zonal optical surface Rs onthe converged-light spot satisfies −0.1π≦φ≦0.1π.

Further, it is preferable that light-converged position fB3 of the lightflux with wavelength λ3 that forms a converged-light spot after passingthrough the ring-shaped zonal optical surface R1 satisfies |fB3|≦5 μm inthe direction of the optical axis for the best image surface of theconverged-light spot formed by the light flux with wavelength λ3 on thethird optical information recording medium.

The objective optical element shown in the present embodiment and theoptical pickup device employing the objective optical element make itpossible to have compatibility for three types of optical informationrecording media.

It is also possible to use an objective optical element wherein S1surface and S2 surface are structured as shown in FIG. 5 by changing thesecond diffraction structure and a surface form of the ring-shaped zonaloptical surface R1, and a vertical spherical aberration diagram is likeone shown in FIG. 7.

As shown in FIG. 7 (a), with respect to a light flux having wavelengthλ1 used for a high density optical disk, its spherical aberration is notchanged within a numerical aperture corresponding to the place wherering-shaped zonal optical surface Rs is formed, namely, a light fluxhaving wavelength λ1 which has passed the ring-shaped zonal opticalsurface Rs is converged on an information recording surface of the firstoptical information recording medium, to be substantially free from anyaberrations.

Further, the spherical aberration is not changed even within a numericalaperture corresponding to the place where ring-shaped zonal opticalsurface R1 is formed, namely, a light flux having wavelength λ1 whichhas passed the ring-shaped zonal optical surface R1 is converged on aninformation recording surface of the first optical information recordingmedium, to be substantially free from any aberrations.

Therefore, it is possible to make spherical aberrations to be almostzero when the total area for numerical aperture NA1 and thereunder isconsidered.

As shown in FIG. 7( b), with respect to a light flux having wavelengthλ2 used for DVD, its spherical aberration is not changed within anumerical aperture corresponding to the place where ring-shaped zonaloptical surface Rs is formed, namely, a light flux having wavelength λ2which has passed the ring-shaped zonal optical surface Rs is convergedon an information recording surface of the second optical informationrecording medium, to be substantially free from any aberrations.

On the other hand, within a numerical aperture corresponding to theplace where ring-shaped zonal optical surface R1 is formed, sphericalaberrations become discontinuous toward the “under” side.

Incidentally, when the combination of the orders of diffracted lightthat makes the diffraction efficiency to be maximum when light fluxeshaving respectively wavelengths λ1, λ2 and λ3 enter is changed bychanging a form of the second diffraction structure, it is also possibleto make the spherical aberration to be discontinuous toward the “over”side.

It is relatively easy to design forms of S1 surface and S2 surface sothat spherical aberrations may be within a range of no trouble inpractical use when the total area corresponding to numerical apertureNA2 is considered.

In this case, a vertical spherical aberration diagram of a light fluxwith wavelength λ3 used for CD shown in FIG. 7( c) is the same as thatshown in the FIG. 6.

Further, as shown in FIG. 8, an objective optical element wherein S1surface and S2 surface shown in the Fourteenth Embodiment are combinedon a plane of incidence may also be used.

To be concrete in explanation, the second total diffractive structure inarea A2 is formed at the position which is deviated toward the opticalinformation recording medium side by a prescribed distance.

Despite this structure, a vertical spherical aberration diagram is thesame as that shown in FIG. 6 or FIG. 7, and it is possible to obtain anoptical pickup device and an objective optical element which havecompatibility for three types of optical information recording media.

Incidentally, an objective optical element wherein S1 surface and S2surface are combined on a plane of emergence may also be used, though anillustration thereof is omitted here.

Fifteenth Embodiment

Compared with the aforementioned Fourteenth Embodiment, an objectiveoptical element shown in the present example is different only on thepoint that the area A2 is constituted with refracting interface 50 asshown in FIG. 9.

FIG. 10 shows an example of a vertical spherical aberration diagram oneach information recording surface of the high density disc, DVD and CD.

As shown in FIG. 10 (a), with respect to a light flux having wavelengthλ1 used for a high density optical disk, its spherical aberration is notchanged within a numerical aperture corresponding to the place wherering-shaped zonal optical surface Rs is formed, namely, a light fluxhaving wavelength λ1 which has passed the ring-shaped zonal opticalsurface Rs is converged on an information recording surface of the firstoptical information recording medium, to be substantially free from anyaberrations.

On the other hand, within a numerical aperture corresponding to theplace where ring-shaped zonal optical surface R1 is formed, sphericalaberrations become discontinuous toward the “under” side.

It is relatively easy to design forms of S1 surface and S2 surface sothat spherical aberrations may be within a range of no trouble inpractical use when the total area corresponding to numerical apertureNA1 is considered.

As shown in FIG. 10 (b), with respect to a light flux having wavelengthλ2 used for DVD, its spherical aberration is not changed within anumerical aperture corresponding to the place where ring-shaped zonaloptical surface Rs is formed, namely, a light flux having wavelength λ2which has passed the ring-shaped zonal optical surface Rs is convergedon an information recording surface of the second optical informationrecording medium, to be substantially free from any aberrations.

On the other hand, within a numerical aperture corresponding to theplace where ring-shaped zonal optical surface R1 is formed, sphericalaberrations become discontinuous toward the “under” side.

It is relatively easy to design forms of S1 surface and S2 surface sothat spherical aberrations may be within a range of no trouble inpractical use when the total area corresponding to numerical apertureNA2 is considered.

In this case, a vertical spherical aberration diagram of a light fluxwith wavelength λ3 used for CD shown in FIG. 10 (c) is the same as thatshown in the FIG. 6( c).

The objective optical element shown in the present embodiment and theoptical pickup device employing the objective optical element make itpossible to have compatibility for three types of optical informationrecording media.

Sixteenth Embodiment

The objective optical element shown in the present example is oneoptical surface 11 (plane of emergence) of the objective opticalelement, as shown in FIG. 11, and it is different from the SeventhEmbodiment in terms of the point that a plurality of ring-shaped zonaloptical surfaces 60 whose centers are on the optical axis are formed onan area through which the light flux with wavelength λ3 that forms aconverged-light spot having numerical aperture NA3 on an informationrecording surface of CD representing the third optical informationrecording medium passes, and of the point that a diffracting ring-shapedzone representing first diffractive structure 30 is formed on each ofarea A1 and area A2 on plane of incidence 12.

To be concrete in explanation, a plurality of ring-shaped zonal opticalsurfaces 60 whose centers are on the optical axis are formed stepwisecontinuously through stepped surface 70.

Among light fluxes having respectively wavelengths λ1, λ2 and λ3, alight flux with wavelength λ3 is given a prescribed optical pathdifference when that light flux passes through each ring-shaped zonaloptical surface 60 so that a phase difference may be generated beforeand after the passing, while, at least one of the light fluxes havingrespectively wavelengths λ1 and λ2 (both of them in the presentembodiment) is not given a prescribed optical path difference when thatit passes through each ring-shaped zonal optical surface 60 so that aphase difference may not be generated before and after the passing.

Now, let it be assumed that Rs represents a ring-shaped zonal opticalsurface including optical axis L and R1 represents a ring-shaped zonaloptical surface that is farthest from the optical axis, among tworing-shaped zonal optical surfaces.

The ring-shaped zonal optical surface Rs is composed of a refractinginterface, and light fluxes having respectively wavelengths λ1, λ2 andλ3 which have passed through the ring-shaped zonal optical surface Rsform converged-light spots respectively on information recordingsurfaces of optical information recording media (“high density opticaldisk”, DVD and CD).

The ring-shaped zonal optical surface R1 is also composed of arefracting interface, and a light flux having wavelength λ3 which haspassed through the ring-shaped zonal optical surface R1 also forms aconverged-light spot on an information recording surfaces of the thirdoptical information recording medium.

FIG. 12 shows an example of a vertical spherical aberration diagram oneach information recording surface of the high density disc, DVD and CD.

As shown in FIG. 12 (a), with respect to a light flux having wavelengthλ1 used for a high density optical disk, its spherical aberration is notchanged within a numerical aperture corresponding to the place wherering-shaped zonal optical surface Rs is formed, namely, a light fluxhaving wavelength λ1 which has passed the ring-shaped zonal opticalsurface Rs is converged on an information recording surface of the firstoptical information recording medium, to be substantially free from anyaberrations.

Further, the spherical aberration is not changed even at the numericalaperture corresponding to the place where ring-shaped zonal opticalsurface R1 is formed, namely, a light flux having wavelength λ1 whichhas passed the ring-shaped zonal optical surface R1 is converged on aninformation recording surface of the first optical information recordingmedium, to be substantially free from any aberrations.

Therefore, it is possible to make spherical aberrations to be almostzero when the total area corresponding to numerical aperture NA1 isconsidered.

Incidentally, according to circumstances, microscopic deviation of alight-converging position by the light flux that has passed eachring-shaped zonal optical surface such as that shown in FIG. 12 (c) mayappear in FIGS. 12 (a) and 12 (b), depending on the ring-shaped zonalstructure on surface S1. In this case, however, vertical sphericalaberration diagrams which are completely free from aberrationsschematically are shown here.

Further, as shown in FIG. 12 (b), with respect to a light flux havingwavelength λ2 used for DVD, its spherical aberration is not changedwithin a numerical aperture corresponding to the place where ring-shapedzonal optical surface Rs is formed, namely, a light flux havingwavelength λ2 which has passed the ring-shaped zonal optical surface Rsis converged on an information recording surface of the second opticalinformation recording medium, to be substantially free from anyaberrations.

Further, the spherical aberration is not changed even within thenumerical aperture corresponding to the place where ring-shaped zonaloptical surface R1 is formed, namely, a light flux having wavelength λ2which has passed the ring-shaped zonal optical surface R1 is convergedon an information recording surface of the second optical informationrecording medium, to be substantially free from any aberrations.

Therefore, it is possible to make spherical aberrations to be almostzero when the total area corresponding to numerical aperture NA2 isconsidered.

Further, as shown in FIG. 12 (c), with respect to a light flux havingwavelength λ3 used for CD, its spherical aberration grows greatergradually toward the “over” side within a numerical aperturecorresponding to the place where ring-shaped zonal optical surface Rs isformed.

On the other hand, within a numerical aperture corresponding to theplace where ring-shaped zonal optical surface R1 is formed, sphericalaberrations become discontinuous toward the “under” side.

It is relatively easy to design forms of S1 surface and S2 surface sothat spherical aberrations may be within a range of no trouble inpractical use when the total area corresponding to numerical apertureNA3 is considered. Incidentally, a light flux having wavelength λ3 whichhas passed the position which is farther from the optical axis than theplace where the ring-shaped zonal optical surface R1 is formed is,becomes a flare.

Therefore, it is not necessary to provide a separate diaphragm for thelight flux having wavelength λ3.

As stated above, an objective optical element shown in the presentembodiment and an optical pickup device employing the objective opticalelement make it possible to achieve compatibility for three types ofoptical information recording media.

Further, optical path difference furnishing structure 90 and diffractingring-shaped zones 80 which are shown respectively in FIG. 13 (a) andFIG. 13 (b) may also be formed on optical surface 12 on one side ofobjective lens 10.

To be concrete, on the objective lens 10, there are formed a pluralityof diffracting ring-shaped zones 80 representing a serrateddiscontinuous surface having a substantial inclination for theprescribed aspheric-shaped optical surface having its center on opticalaxis L, and further, on the optical surface of each diffractingring-shaped zone 80, there is formed optical path difference furnishingstructure 90 composed of staircase-shaped discontinuous surfaces (steps)91 in the direction of an optical axis which furnish a prescribedoptical path difference to the light flux that passes through thediffracting ring-shaped zones 80.

One-dot-chain lines in FIGS. 13 (a) and 13 (b) indicate an opticalsurface (optical surface in a shape of a prescribed aspheric surface)which is in a form of an imaginary aspheric surface obtained byconnecting vertexes of diffracting ring-shaped zones 80 as stated above,while, two-dot-chain lines indicate an outer form of the known serrateddiffracting ring-shaped zones 80 in a shape of concentric circles whichare formed in a way that a thickness is increased gradually as aposition for the thickness recedes from optical axis L which serves asthe center.

Solid lines indicate a form of the actual lens including an outer formof step 91 that is formed on an optical surface of the diffractingring-shaped zones 80 and furnishes a prescribed optical path differenceto the light flux passing through each diffracting ring-shaped zones 80.

Depth d1 of the step 91 (length in the direction of optical axis L) isnearly the same as a value expressed by λ2/(n−1) when n represents therefractive index of the objective lens for wavelength λ2, for example,and it is established to be in the length wherein an optical pathdifference equivalent substantially to one wavelength λ2 is generatedbetween a light flux with wavelength λ2 passing through one step and alight flux with wavelength λ2 passing through the adjoining step, and noslippage of a wave front is caused.

Further, a form of surface 91 a (optical functional surface) of eachstep approximates a form wherein a form of the surface of serrateddiffracting ring-shaped zones 80 shown with two-dot chain lines in thediagram is split by sections corresponding to each step 91 to be movedin parallel in the direction of optical axis L.

As stated above, owing to the optical path difference furnishingstructure 90 that is equipped on its optical surface with step 91 havinga prescribed depth, there is given a function to furnish a prescribedoptical path difference to a light flux passing through objective lens10, and owing to the form of surface 91 a of each step whereindiffracting ring-shaped zones 80 is split by sections corresponding tostep 91 to be moved in parallel in the direction of optical axis L,there is given a function to extract diffracted light of the diffractionorder that causes the maximum diffraction efficiency for light fluxeshaving respectively wavelengths λ1-λ3.

For example, when the light flux with wavelength λ1 (650 nm) enters anobjective lens, the light flux with wavelength λ1 is subjected todiffraction effect by the diffracting ring-shaped zones 80, and eachlight flux is substantially given a phase difference of 780 nm−650nm=130 nm, namely of ⅖π radian, after passing through areas A-E in FIG.13 (b), resulting in receiving of diffraction effect caused by changesof a phase of the light flux with wavelength λ1.

On the other hand, when the light flux with wavelength λ2 (780 nm)enters, the light flux with wavelength λ2 is subjected to diffractioneffect by the diffracting ring-shaped zones 80, and each light flux isgiven a phase difference corresponding to one wavelength λ2 afterpassing through areas A-E in FIG. 5, and a phase difference causedbetween light fluxes passing through areas A-E becomes almost zero.Therefore, the light flux with wavelength λ2 is not diffractedsubstantially by the optical path difference furnishing structure 90 tobe transmitted as it is.

Since the diffraction order of a light flux with each wavelength can bechanged substantially by two steps of the diffracting ring-shaped zones80 and the optical path difference furnishing structure 90, it ispossible to obtain diffracted light having a sufficient amount of lightcorresponding to the types of optical information recording media, bychanging the diffraction order of each light flux according tocircumstances. It is also possible to increase the degree of freedom fordesign for diffraction efficiency and the diffraction order.

EXAMPLE

Next, the first example of the optical pickup device and the opticalelement shown in the aforementioned embodiment will be explained.

In the present example, let it be assumed to use an objective lenswherein surface S1 and surface S2 are combined on the plane of incidenceside such as that shown in FIG. 8.

To be concrete in explanation, a plane of incidence of an objective lensrepresenting a single and two-sided aspheric lens is divided into No. 2surface whose height h from optical axis L is 1.45 mm or more, No. 2′surface whose height is not less than 1.1 mm and is less than 1.45 mmand No. 2″ whose height is less than 1.1 mm, as shown in FIG. 14.

On No. 2′ surface and No. 2″ surface, there are formed plural serratedand discontinuous diffracting ring-shaped zones 80 having substantialinclination for an optical surface in a form of a prescribed asphericsurface, and on the optical surface of each diffracting ring-shaped zone80, there is formed optical path difference furnishing structure 90 thatgives a prescribed optical path difference to a light flux passingthrough the diffracting ring-shaped zones 80 and is composed ofstaircase-shaped discontinuous surfaces (steps) 91 in the direction ofan optical axis, and each step 91 formed on one diffracting ring-shapedzone 80 is in a shape that is more protruded toward the light sourceside as a position of the shape recedes from optical axis L.

On the No. 2 surface, there are formed only serrated ring-shaped zones.

Then, light fluxes having respectively wavelengths λ1, λ2 and λ3 whichpass through No. 2′ surface and No. 2″ surface emerge after beingsubjected to diffracting effect by the aforementioned diffractingring-shaped zone 80 and the optical path difference furnishing structure90 so that diffracted light with diffraction order m−2, n−1 and k=0which make the diffraction efficiency to be maximum.

To be concrete in explanation, under the assumption that no optical pathdifference furnishing structure 90 is provided, the optical surface ofthe diffracting ring-shaped zone 80 diffracts so that second (=mB1)order diffracted light of the light flux with wavelength λ1 may have themaximum diffraction efficiency, first (=mB2) order diffracted light ofthe light flux with wavelength λ2 may have the maximum diffractionefficiency, and first (=mB3) order diffracted light of the light fluxwith wavelength λ3 may have the maximum diffraction efficiency.

Further, the optical path difference furnishing structure 90 gives tothese diffracted light an optical path difference by which the second(=m) order diffracted light of the light flux with wavelength λ1 has themaximum diffraction efficiency, an optical path difference by which thefirst (=n) order diffracted light of the light flux with wavelength λ2has the maximum diffraction efficiency, and an optical path differenceby which the 0^(th) (=k) order diffracted light of the light flux withwavelength λ3 has the maximum diffraction efficiency.

In other words, an optical path difference is given to each light fluxso that the following expressions are satisfied,m=2−0=2n=1−0+0=1k=1−0−1=0under the assumption that mB1=2, mB2=1, mB3=1 and mD=0, in the followingexpressions (1)-(3).m=mB1−mD  (1)n=mB2−mD+(−1,0 or 1)  (2)k=mB3−mD+(−1,0 or 1)  (3)

Further, light fluxes having respectively wavelengths λ1, λ2 and λ3which pass through No. 2 surface emerge after being subjected todiffracting effect by the aforementioned diffracting ring-shaped zone 80so that diffracted light with m=2, n=1 and k=1 may be obtained.

As stated above, for light fluxes having respectively wavelengths λ1 andλ2, diffracting effects are given to them when they pass respectivelythrough No. 2 surface, No. 2′ surface and No. 2″ surface so thatdiffracted light with diffraction orders m=2 and n=1 may be obtained,while, for light flux with wavelength λ3, diffracting effects are givento it so that diffracted light with different diffraction orders may beobtained when the light flux passes through No. 2′ surface and No. 2″surface and when the light flux passes through No. 2 surface, thus, itis possible to make the light flux (first order diffracted light) havingwavelength λ3 that has passed through No. 2 surface to be a flarewithout making the light flux to be converged on an informationrecording surface of CD.

Lens data of the objective lens are shown in Table 1 and Table 2.

TABLE 1 Example 1 lens data Focal length of objective lens f1: 3.1 mmf2: 3.26 mm f3: 3.57 mm Image side numerical aperture NA1: 0.65 NA2:0.62 NA3: 0.40 Diffraction order n1: 0 n2: 1 n3: 0 Magnification m1: 0m2: 0 m3: 0 i-th surface Ri di (407 nm) ni (407 nm) di (655 nm) ni (655nm) di (785 nm) ni (785 nm) 0 ∞ ∞ ∞ 1 ∞ 0.1 0.1 0.1 (aperture (Φ4.03 mm)(Φ4.03 mm) (Φ2.89 mm) diameter) 2 2.05949 −0.004843 1.559806 −0.0048431.540725 −0.004843 1.537237 2′ 1.65821 −0.025414 −0.025414 −0.025414 2″2.11730 1.730000 1.730000 1.730000 3 −14.46196 1.71 1.85 1.0 1.79 1.0 4∞ 0.60 1.0 0.60 1.57752 1.20 1.57063 5 ∞ 1.61869 * di represents adisplacement from i-th surface to (i + 1)-th surface. (provided that,d2, d2′ represents a displacement up to 2″-th surface)

As is shown in Table 1, the objective lens of the present example is setto focal length f=3.1 mm and image-side numerical aperture NA1=0.65 forwavelength λ1=407 nm, to focal length f2=3.26 mm and image-sidenumerical aperture NA2=0.62 for wavelength λ2=655 nm and to focal lengthf3=3.57 mm and image-side numerical aperture NA3=0.40 for wavelengthλ3=785 nm.

Further, magnifications m1-m3 respectively for light fluxes with λ1-λ3are almost zero, which constitutes the structure of an infinite systemwherein parallel light enters the objective lens.

Surface No. 2, 2′ and 2″ in Table 1 show respectively No. 2 surface of1.45 mm≦h, No. 2′ surface of 1.1 mm≦h<1.45 mm and No. 2″ surface ofh<1.1 mm among planes of incidence of the objective lens, and SurfaceNo. 3 and 4 show respectively a surface of a protective substrate and arecording layer of the optical information recording medium. Further, Rirepresents a radius of curvature, di represents a displacement from thesurface i to the surface i+1 in the direction of an optical axis and nirepresents a refractive index of each surface.

No. 2 surface, No. 2′ surface, No. 2″ surface and No. 3 surface of theobjective lens are formed to be aspheric surfaces which are prescribedby the expression wherein coefficients shown in Table 1 and Table 2 aresubstituted in the following expression (Numeral 1) and are aroundoptical axis L to be symmetrical with respect to the axis.

$\begin{matrix}{{{Aspherical}\mspace{14mu}{configuration}\mspace{14mu}{formula}}{{X(h)} = {\frac{\left( {h^{2}/R} \right)}{1 + \sqrt{1 - {\left( {1 + \kappa} \right)\left( {h/R} \right)^{2}}}} + {\sum\limits_{i = 0}^{9}\;{A_{2i}h^{2\; i}}}}}} & \left( {{Numeral}\mspace{14mu} 1} \right)\end{matrix}$

In the expression, X (h) represents an axis in the direction of anoptical axis (the direction of travel is positive), κ represents theconstant of the cone and A_(2i) represents an aspheric surfacecoefficient.

TABLE 2 Aspherical surface data Second surface(1.45 mm ≦ h) Asphericalsurface κ −4.8542 × E−1 coefficient A4 −2.9942 × E−3 A6 −9.4811 × E−5 A8+4.4741 × E−4 A10 −1.2388 × E−4 A12 −1.2201 × E−5 A14 −7.2281 × E−7Optical path C2 −1.9949 × E+1 difference function C4 −8.1237 × E−1 C6+5.6955 × E−1 C8 −1.6478 × E−1 C10 +1.8361 × E−2 2′-th surface(1.1 mm ≦h < 1.45 mm) Aspherical surface κ −1.8050 coefficient A4 −2.3866 × E−2A6 +4.9979 × E−3 A8 +7.6653 × E−3 A10 −1.6235 × E−4 A12 −1.9529 × E−3A14 +4.6746 × E−4 Optical path C2 −1.9615 × E+1 difference function C4−3.4498 × E−1 C6 −6.7775 × E−1 C8 +6.2791 × E−1 C10 −1.5009 × E−1Ssecond surface(h < 1.1 mm) Aspherical surface κ −5.6043 × E−1coefficient A4 −1.2469 × E−2 A6 −3.4236 × E−3 A8 −1.3601 × E−3 A10+1.4240 × E−5 A12 +1.7673 × E−5 A14 −1.2485 × E−6 Optical path C2−1.9785 × E+1 difference function C4 −9.7147 × E−1 C6 +1.6560 C8 −1.6829C10 −5.9962 × E−1 Third surface Aspherical surface κ −7.3166 × E+2coefficient A4 −1.2546 × E−2 A6 +1.1479 × E−2 A8 −5.0025 × E−3 A10−1.2263 × E−3 A12 −1.6898 × E−4 A14 +1.0226 × E−5

Further, a pitch of the ring-shaped zones is prescribed by theexpression wherein a coefficient shown in Table 2 is substituted for theoptical path difference function of Numeral 2.

$\begin{matrix}{{{Optical}\mspace{14mu}{path}\mspace{14mu}{difference}\mspace{14mu}{function}}{\phi_{(b)} = {\left( {\sum\limits_{i = 0}^{5}\;{B_{2i}h^{2i}}} \right) \times m_{D} \times \frac{\lambda}{\lambda\; B}}}} & \left( {{Numeral}\mspace{14mu} 2} \right)\end{matrix}$m_(D): Diffraction order in the case that there is not provided aoptical path difference providing structure.λ: used wavelengthλ_(B): blazing wavelength for diffraction

(λ_(B)=1 mm in the example)

In the expression, B_(2i) represents a coefficient of the optical pathdifference function.

FIG. 15 is a graph showing an amount of fluctuation of verticalspherical aberration and a numerical aperture (NA) in the occasion wherea wavelength of a light flux having wavelength λ1 (407 nm) used for ahigh density disc (AOD) fluctuates by ±1 nm from 407 nm.

Since the amount of fluctuation of a wavelength caused by mode-hop orthe like is about 1 μm as a rule, it is understood that an amount offluctuation of vertical spherical aberration is kept within a range ofno trouble in practical use with the aforementioned range, and asufficient function to correct chromatic aberrations is provided.

FIG. 16 shows wave-front aberration and diffraction efficiency for eachof light fluxes having respectively wavelengths λ1 (AOD), λ2 (DVD) andλ3 (CD), and FIG. 17-FIG. 19 show graphs each showing light-convergedposition fB and a numerical aperture of each of light fluxes havingrespectively wavelengths λ1-λ3.

From FIG. 16-FIG. 19, it is understood that wave-front aberration ofeach light flux is kept to a diffraction limit of 0.07 λrms or less anda sufficient function to correct chromatic aberrations is provided. Itis further understood that a sufficient diffraction efficiency isprovided to be used for recording and/or reproducing information foreach optical information recording medium.

Next, there will be explained Second Example for the optical pickupdevice and the optical element shown in the aforementioned embodiment.

In the present example again, let it be assumed that an objective lenswherein surface S1 and surface S2 are combined on the plane of incidenceside like that shown in FIG. 8 is used.

To be concrete in explanation, a plane of incidence of an objective lensrepresenting a single and two-sided aspheric lens is divided into No. 2surface whose height h from optical axis L is 1.45 mm or more, No. 2′surface whose height is not less than 1.1 mm and is less than 1.45 mmand No. 2″ whose height is less than 1.1 mm (area A2).

On area A1, there are formed a plurality of ring-shaped zones as thefirst diffractive structure, and on area A2, there are formedring-shaped zones as the second diffractive structure. The ring-shapedzones are formed also on No. 2 surface.

Light fluxes having respectively wavelengths λ1, λ2 and λ3 whichrespectively pass No. 2 surface, No. 2′ surface and No. 2″ surface aresubjected by the first and second diffractive structures to diffractioneffects to emerge, so that diffracted light respectively withdiffraction orders m=3, n=2 and k=2 for the maximum diffractionefficiency may be obtained.

Lens data of the objective lens are shown in Table 3 and Table 4.

TABLE 3 Example 2 lens data Focal length of objective lens f1: 3.1 mmf2: 3.19 mm f3: 3.18 mm Image side numerical aperture NA1: 0.65 NA2:0.63 NA3: 0.45 Diffraction order n1: 3 n2: 2 n3: 2 Magnification m1: 0m2: 0 m3: 0 i-th surface Ri di (407 nm) ni (407 nm) di (655 nm) ni (655nm) di (785 nm) ni (785 nm) 0 ∞ ∞ ∞ 1 ∞ 0.1 0.1 0.1 (aperture (Φ4.03 mm)(Φ4.03 mm) (Φ2.89 mm) diameter) 2 2.04672 0.005188 1.559806 0.0051881.540725 0.005188 1.537237 2′ 2.19683 0.013857 0.013857 0.013857 2″1.99496 1.730000 1.730000 1.730000 3 −13.44828 1.72 1.78 1.0 1.38 1.0 4∞ 0.60 1.0 0.60 1.57752 1.20 1.57063 5 ∞ 1.61869 * di represents adisplacement from i-th surface to (i + 1)-th surface. (provided that,d2, d2′ represents a displacement up to 2″-th surface)

As is shown in Table 3, the objective lens of the present example is setto focal length f1=3.1 mm and image-side numerical aperture NA1=0.65 forwavelength λ1=407 nm, to focal length f2=3.19 mm and image-sidenumerical aperture NA2=0.63 for wavelength λ2=655 nm and to focal lengthf3=3.18 mm and image-side numerical aperture NA3=0.45 for wavelengthλ3=785 nm.

Further, magnifications m1-m3 respectively for light fluxes with λ1-λ3are almost zero, which constitutes the structure of an infinite systemwherein parallel light enters the objective lens.

No. 2 surface, No. 2′ surface, No. 2″ surface and No. 3 surface of theobjective lens are formed to be aspheric surfaces which are prescribedby the expression wherein coefficients shown in Table 3 and Table 4 aresubstituted in the Numeral 1 and are around optical axis L to besymmetrical with respect to the axis.

TABLE 4 Aspherical surface data Second surface(1.45 mm ≦ h) Asphericalsurface κ −5.4210 × E−1 coefficient A4 −1.6156 × E−3 A6 +5.2867 × E−4 A8+4.5232 × E−4 A10 −1.9450 × E−4 A12 −1.9945 × E−5 A14 −8.1755 × E−7Optical path C2 −5.6884 difference function C4 −2.3034 C6 +5.8398 × E−1C8 −1.1111 × E−1 C10 +5.2102 × E−3 2′-th surface(1.1 mm ≦ h < 1.45 mm)Aspherical surface κ −3.7554 × E−1 coefficient A4 −7.9273 × E−3 A6+2.1214 × E−2 A8 −3.0263 × E−3 A10 −9.9477 × E−3 A12 +5.9443 × E−3 A14−1.0283 × E−3 Optical path C2 −5.5699 difference function C4 −2.5038 C6−7.8130 × E−2 C8 +5.7889 × E−1 C10 −2.0441 × E−1 Second surface(h < 1.1mm) Aspherical surface κ +6.8230 × E−1 coefficient A4 −3.5343 × E−2 A6+2.8071 × E−2 A8 −3.4333 × E−2 A10 +1.1079 × E−2 A12 +1.7674 × E−5 A14−1.2485 × E−6 Optical path C2 −5.7100 difference function C4 −7.2912 C6+1.5277 × E−1 C8 −1.5615 × E−1 C10 +5.6948 Third surface Asphericalsurface κ −1.2944 × E+2 coefficient A4 −3.6832 × E−3 A6 +1.0114 × E−2 A8−5.6473 × E−3 A10 +1.4453 × E−3 A12 −1.7972 × E−4 A14 +8.5800 × E−6

Further, a pitch of the ring-shaped zones is prescribed by theexpression wherein a coefficient shown in Table 4 is substituted for theoptical path difference function of the aforesaid Numeral 2.

FIG. 20 is a graph showing an amount of fluctuation of sphericalaberration and a numerical aperture (NA) in the occasion where awavelength of a light flux having wavelength λ1 (407 nm) used for a highdensity disc (AOD) fluctuates by ±1 nm from 407 nm.

Since the amount of fluctuation of a wavelength caused by mode-hop orthe like is about 1 μm as a rule, it is understood that an amount offluctuation of vertical spherical aberration is kept within a range ofno trouble in practical use with the aforementioned range, and asufficient function to correct chromatic aberrations is provided.

FIG. 21 shows wave-front aberration and diffraction efficiency for eachof light fluxes having respectively wavelengths λ1 (AOD), λ2 (DVD) andλ3 (CD), and FIG. 22-FIG. 24 show graphs each showing light-convergedposition fB and a numerical aperture of each of light fluxes havingrespectively wavelengths λ1-λ3.

From FIG. 21-FIG. 24, it is understood that wave-front aberration iskept to a diffraction limit of 0.07 λrms or less and a sufficientfunction to correct chromatic aberrations is provided. It is furtherunderstood that a sufficient diffraction efficiency is provided to beused for recording and/or reproducing information for each opticalinformation recording medium.

In the optical pickup device and the optical element relating to theinvention, the optical pickup device is arranged on the common opticalpath for the first, second and third light sources, and a diffractiveoptical element having the first diffractive structure is provided, andwhen conducting reproducing and/or recording of information for thefirst, second and third optical information recording media, all lightfluxes are made to enter the diffracting light optical element at thesubstantially same angle.

Therefore, the optical paths for light respectively with the first-thirdwavelengths are mostly the same, thus, various types of optical elementsconstituting the optical pickup device have only to be arranged tocorrespond to the common optical path, thereby, the structure of theoptical pickup device can be simplified and the number of parts of thedevice can be reduced.

Further, all the light fluxes are made to enter the diffractive opticalelement as substantially infinite parallel rays.

Therefore, it is possible to prevent that image height characteristicsare worsened in the case of tracking to move an objective opticalelement for the optical information recording medium, and to inhibitoccurrence of various aberrations such as coma and astigmatism.

It is further possible to inhibit spherical aberration caused bytemperature changes.

1. An optical pickup apparatus, comprising: a first light source to emita light flux of a wavelength λ1 for conducting at least one of recordingand reproducing information for a first optical information recordingmedium having a protective substrate having a thickness t1; a secondlight source to emit a light flux of a wavelength λ2 (λ1<λ2) forconducting at least one of recording and reproducing information for asecond optical information recording medium having a protectivesubstrate having a thickness t2 (t1≦t2); a third light source to emit alight flux of a wavelength λ3 (λ2<λ3) for conducting at least one ofrecording and reproducing information for a third optical informationrecording medium having a protective substrate having a thickness t3(t2<t3); and an objective lens comprising a diffractive structure;wherein an infinite parallel light flux comes into the objective lenswhen at least one of recording and reproducing information is conductedfor the first, second and third optical information recording mediums,wherein the objective lens forms a converged-light spot sufficient forconducting at least one of reproducing and recording information atleast for the first optical information recording medium and generatesdifferent order diffracted-light ray for the light flux of thewavelength λ2 or the light flux of the wavelength λ3 from the order of adiffracted-light ray of the light flux of the wavelength λ1, wherein thediffractive structure of the objective lens comprises superimposed firstand second structures, wherein the first structure generates 2nd orderdiffracted-light ray for the light flux of the wavelength λ1, 1st orderdiffracted-light ray for the light flux of the wavelength λ2, and 1storder diffracted-light ray for the light flux of the wavelength λ3; andwherein the second structure generates 0th order diffracted-light rayfor the light flux of the wavelength λ1, 0th order diffracted-light rayfor the light flux of the wavelength λ2, and 1st order diffracted-lightray for the light flux of the wavelength λ3.
 2. The optical pickupapparatus of claim 1, wherein the objective lens is a single lens. 3.The optical pickup apparatus of claim 1, wherein the objective lens isplural lenses.
 4. An objective lens for an optical pickup apparatus, theoptical pickup apparatus comprising: a first light source to emit alight flux of a wavelength λ1 for conducting at least one of recordingand reproducing information for a first optical information recordingmedium having a protective substrate having a thickness t1; a secondlight source to emit a light flux of a wavelength λ2 (λ1<λ2) forconducting at least one of recording and reproducing information for asecond optical information recording medium having a protectivesubstrate having a thickness t2 (t1≦t2); a third light source to emit alight flux of a wavelength λ3 (λ2<λ3) for conducting at least one ofrecording and reproducing information for a third optical informationrecording medium having a protective substrate having a thickness t3(t2<t3); and the objective lens comprising a first diffractivestructure; wherein the objective lens forms a converged-light spotsufficient for conducting at least one of reproducing and recordinginformation at least for the first optical information recording mediumand generates different order diffracted-light ray for the light flux ofthe wavelength λ2 or the light flux of the wavelength λ3 from the orderof a diffracted-light ray of the light flux of the wavelength λ1;wherein the diffractive structure of the objective lens comprisessuperimposed first and second structures; wherein the first structuregenerates 2nd order diffracted-light ray for the light flux of thewavelength λ1, 1st order diffracted-light ray for the light flux of thewavelength λ2, and 1st order diffracted-light ray for the light flux ofthe wavelength λ3; and wherein the second structure generates 0th orderdiffracted-light ray for the light flux of the wavelength λ1, 0th orderdiffracted-light ray for the light flux of the wavelength λ2, and 1storder diffracted-light ray for the light flux of the wavelength λ3. 5.The optical pickup apparatus of claim 4, wherein the objective lens is asingle lens.